You gave money to the priest who performed the ceremony - you didn't hire him.
Same as you gave money to have your wedding performed in the church - you didn't rent it.
That's all people are saying. Not telling you what happened at your wedding but that, even if you weren't aware (maybe because it didn't come up) you didn't rent the church or have the ability to restrict access. It's just not what the RCC does, anyplace.
Same as you give money to the priest who performs the wedding, but you're not hiring him. It likely didn't come up either but you couldn't have told him to say something he wouldn't say or rearranged the mass to suit yourselves or even get a priest to marry a couple they don't feel they should marry (because the couple didn't do well in premarital counseling or what have you). If he was for hire, all those things would be things that could be done - you pay, you tell someone what to say, but you're not hiring priests, it doesn't work that way. Same as you're not renting out the church and gaining the ability to restrict access to it during your mass.
People show up, for a variety of reasons, and that's just the way it is.
As to the poll, in general, I think if you know people were invited and you were not, it's rude to show up (unless it's like you know because the wedding was posted in the bulletin - I mean if you know them and they didn't invite you, for whatever reason, stay away). If it's a wedding in a Catholic or Episocpalian or etc., church or temple that's open - then I think it's fine to walk in and sit in the back very unobtrusively. The Church is supposed to be a community and it's a community event. Same as plenty of random people end up at christenings, baptisms, funeral masses, etc. - you make a donation for those too, and plenty of people you've never seen are there, because you aren't renting the church.