OP- I answered other. We've definitely brought in jarred baby foods when our kids were in the 6-18 month age range, and we've also brought toddler nibbles in. I figure that a restaurant would much rather my little one have a few cheerios to occupy themselves while we wait than to hear a meltdown.
I've also brought things for me for my allergies.
I do draw the line at bringing stuff from another restaurant though, it just seems tacky.
My DMIL has a very specific diet due to allergies. Not all restaurants can or will accommodate. She has taken food from home into a restaurant when we go out as a group. We always check ahead of schedule of course and we have never had a restaurant forbid it. Especially those that won't accommodate her diet.
That's my personal situation as well. I am allergic to gluten, tree nuts and strawberries, and can't tolerate meat or raw veggies. Some places are just too difficult to manage with those restrictions.
If I'm eating out with just my immediate family (DH and kids) we only go somewhere we know can accommodate my needs, but other times it's an event or a large group and I can't make everyone change for my needs.
We've had two situations recently- one was a 90th birthday party for DH's grandmother. I did call the kitchen ahead of time but they were way too clueless about just the gluten allergy for me to feel safe ordering something there- so I went and just sat there while everyone else ate. It sucks having to do that and it invites so much BS from other people ("You're pregnant, you have to eat!", "Why don't you just take the stuffed chicken and peel the breading off", etc..) - but I am the one who has to deal with getting sick/having an allergic reaction if things don't go right.
In another situation we were going out with a large group of friends. It's really not fair for me to ask 15 other people who want to go to a nice local place to change it up for me. So, since it was an Italian restaurant, I called, spoke with the manager and got the okay to bring my own rice pasta cooked at home, in a tupperware and they just brought me sauce (which I paid the price of the full dinner for a side of sauce, no salad, rolls or anything else

)
Get your coat and hat on because that is just the begininning of the snowflake blizzard. I think it is wrong and lazy of the parents. Simply say NO! If we are going out to dinner- yes, even for sushi- my kids eat off the menu. They will also have to try the new food. Of course if they don't like it they don't have to eat a plate of it but they do have to try it. There is always something on a menu that they like. It's not like restaurants serve live animals that you have to beat and kill for dinner with a side of brussel sprouts. The Mom and Dad you talk about are lazy parents who will indulge snowflake no matter what because they simply either can't be bothered to parent and teach their kids something or they think their child is a snowflake.
YMMV.

Like I said above, I've made accommodations for my allergies, but when it comes to my kids we are very much like you. There is always SOMETHING that they will eat- and I have two really picky kids. We always find one or two things they will eat, even if it means an assortment of appetizers and side dishes.
We've also left the two picky ones at home and taken the adventurous eater out with just DH and I- they enjoy staying with Grandma and we enjoy not being limited by them. I think that it's helped them to realize that they are missing out though when the other one comes in gushing about what a great time she had- especially for the older one who has to listen to "the baby" getting to do something she doesn't.
Yes if it's for allergies. My youngest is allergic to eggs and unfortunately we are finding that people don't get this allergy. Peanuts and gluten then seem to get. Eggs, no. They think it's OK if there is no egg but it was cooked in a vessel that cooked other egg. or they simply don't know.
Nah, they are bad with the gluten too. I've had Italian places that told me they would cook rice pasta if I brought it. When pressed further, they were going to cook it in the same water they cook the regular pasta in

I've also been told to peel breading off chicken and then it would be gluten free (um.....no). You get the point, it's just not worth the risk if you don't feel 100% comfortable that the person really appreciates what they are dealing with.
I think a follow-up poll of 'who makes more than one meal for dinner at home' might be in order. I'm guessing that the ones who will let their child choose when going out would also let them choose the dinner menu at home.
I've had someone bring PB&J for their child to our house for dinner. Even though different than a restaurant - I thought even that was strange! I have kids too and had some 'kid friendly' stuff. But apparently all their son would eat at that time was PB&J. I guess if you let him have it every time...that WOULD be the only thing he might want...
LOL, I often make more than one meal- one regular foods for those who can and one that accommodates my allergies. No reason why everyone else should have to give up their favorites because of me. They've also all had takeout pizza from a wonderful local place while I have something safe at home. DH suggested at one point making the whole house gluten free but I cannot justify the added expense- for example, I can buy a nice loaf of bread for $2, or heck, a really nice loaf from Great Harvest bakery for $3, but a tiny loaf of GF bread is $7!!!
We've brought stuff for our kids places too- maybe your friends thought they were making things easier on you by not imposing?? I know I did that for myself this year for Christmas Eve and Christmas- I figured when people are hosting a large gathering the last thing they needed was the extra stress of figuring out my special diet on top of everything else. In my case it's nice because then I am also assured that my foods are safe and prepared in a safe environment. I know that's not the issue with a pb&J for a kid but I imagine the parents felt they were imposing if they asked.