Sometimes a bride gets wrapped up in her special day a bit too much and forgets a little how to act like a human being.
This is funny, because my sister is the LEAST bridezilla-like person I know. She refused to have a bridal shower, engagement party, or bachelorette party because she didn't want her friends to feel obligated to buy gifts/spend money on her, and their registry is very modest (Target). I also fail to see how someone is "forgetting to act like a human being" when every single etiquette source I've been able to find (other than opinions on the DIS

) says that what they're doing is perfectly proper?
Clearly everyone has different takes on who is in the "wrong" and everyone is entitled to their opinion. Personally i think both sides are a little bit rude. But this quote is the one that jumps out to me...
Basically you are saying there IS room for these unexpected +1s but it's just preventing someone from the "B list" from getting a late invite. If the B list guests are that important they should have been on the A list to start.
In addition to cost concerns, the space at the venue is not unlimited - there is a hard and fast limit (120, so not huge), and there are big families on both sides. If they had invited everyone right up front and gotten more "yes" responses than expected, they would have been screwed - in fact, they have gotten more "yes" responses than expected in those that were sent out. The people on the "B List," as you put it, are coworkers who my sister is friendly with and who have told her that they would love to attend the wedding if they can.
Yeah. they have such a good handle on it that two or three plus ones are going to break them. Such a good handle indeed.
Nowhere did I say it was going to "break them." I said "They will be able to squeeze these few in if they have to but if they get many more it will be a problem, and it is taking away from a few additional people that they would like to invite if they have space." Their venue does not have an unlimited capacity...I hardly think that's unusual, and if people take it upon themselves to invite additional guests, that obviously screws with their numbers.
So your sister, who wants to save money, can just invite a person who has to fly out for the wedding and fell it is OK to have them be alone the whole time?
And yet, it seems to be ok to ask someone to fly in and come alone and share their day with a bunch of strangers, all for a sum total of 5 minutes they're actually going to be sharing with (instead of watching from a distance) the bride and groom.
Nowhere has the OP said that these guests would not know other people at the reception, and I think that is a major distinguishing factor. I have been to more weddings than I can count as a single, and that includes those where I was granted a +1 but chose not to bring one if I wasn't in a relationship. I have been to very few weddings that didn't also include friends and acquaintances for me to spend time with. At a coworker's wedding, I was seated for dinner with people I didn't know. Was I thrilled to make small talk for an hour, no, but I spent the rest of the evening with my coworkers anyway, and in the grand scheme of things it was hardly that big of a deal.
I'm not talking about situations where the guest doesn't know many people or larger weddings where cost, space, etc. are not as limited. In cases like those, I do think a +1 is the more considerate option. However, if someone's invited to a smaller wedding, they presumably know the bride and/or groom fairly well and would probably know through the grapevine that it's a limited guest list. I would never expect the couple to exclude people they know to provide me a +1. If I found out that someone else was given a +1, I would assume there was some reason behind it and it wasn't a statement on my value as a friend. I find it sad that people can't manage one night on their own to celebrate a friend's wedding.
As someone who is single, I still don't understand the assumption that a single person is incapable of attending a social event without an escort...but in any event, my close friend who rsvp'd for her (platonic) roommate as her plus one knows me as well as several of my sister's childhood friends and members of our family as we grew up together. She lives a few hours away from where the wedding will be held, so no flight necessary. The person who has already bought airfare/hotel for herself plus her uninvited guest is a distant cousin on the groom's side - she will know dozens of people at the wedding, including her parents. My understanding is that her "plus one" is a guy she has been dating about a month. They were hardly going to be sitting alone in a corner at the reception if they came by themselves. I don't believe there are any single guests who did not get a plus one who would be flying in and would not know anyone else at the wedding - most of the people traveling are either family or close college friends.
That is a very selective criteria in which some unmarried guests will receive a +1 and some will not. As I mentioned, there are obvious times when special consideration is given to special guests like the parents of the bride or the wedding party, but beyond that there is a risk of setting up a very visible "cool kid's club".
For that matter, I wonder how much research was put into each invite before deciding who was and wasn't 'seriously' dating someone? Like I said this is an opportunity for communication between the host and guest, not condemnation. In this particular case there was 3 or so obvious cases of miss-communication. That's a big part of the benefit of RSVP, that you get to sort some of these things out before hand. And you learn, in one case, that someone the host wanted at their wedding is willing to fly across the country for them at great personal cost of precious time and money and the complaint here is that she's bringing an escort that will cost the host an extra ... what $75?
As I said, it's not a large wedding. They know their friends/family, so there was no research needed and those unmarried guests who were invited as couples were invited in their own right, because my sister and her fiance know them and consider them friends.
Putting aside the space constraints, I agree - $75 is not a big deal...for me. However, it is for my sister and her fiance, who are paying for this wedding themselves. They're not cheap - wedding will have open bar, etc., but they don't have unlimited funds. A wedding invitation is an invitation, not a command, so if someone is really so offended that they can't bring a random guest, they're free to decline.