I have to say this kind of stuff drives me crazy. I would absolutely never allow my children to be rude and disruptive to people like that.
There is this one mom at our gym that sits her kid right down in the middle of a table of parents having a discussion and turns the video on The kids device up full volume and then just walks away. Really? How about some headphones or go put her in a chair in the corner at least.
We have another who yells in excited baby talk to her one year old for the entirely of her dds class. That one is a joy.
We have the bleacher jumpers too. I don't care if the kid is two or ten, this should be halted immediately. It is disruptive and rude to the other people there trying to quietly watch or have a conversation or get some work done or read their child a book quietly while they wait. One time a woman's computer got jumped right onto the floor and broke, so there is more than just our sanity at stake here.
And I don't want to hear it is hard and what am I supposed to do with them while they have to sit there for that long. Not the rest of our problems. I have been managing this entertain the sibling thing for many years. My youngest son has literally grown up in the gym lobby and at meets with all the other little brothers and not once have I ever allowed him to be disruptive without calling him on it immediately. Bring activities, a device, books, coloring, homework, whatever, but do it quietly.
With that said, I think it is very difficult to do anything about. One time I did catch the dad's eye of the girl jumping up and down the bleachers and gave him the "really?" Face and he did make her stop. I have said something softly to someone before. I find being quiet and very sweet seems to work the best but you have to be prepared for whatever their reaction might be.
I don't know, it is definitely a tough one. I just don't understand why people find it so acceptable to be so completely inconsiderate of one another. Don't even get me started on people's driving habits. LOL!!!