And I will correct people who do it if there is a situation that I am able to handle....people may or may not like it. But, adults are not exempt from proper behavior, either.
My son went through a phase where he was very interested in the button in elevators that calls the Fire Department. He never pushed it -- never even came close. But, he wanted a nice look at it, and he wanted to talk about it. He loved elevators and escalators and I would often use them as behavioral rewards, so we visited them frequently. It was not unusual during that time for people to tell him not to touch the button. And, I invariably had to tell them that I had it under control. After all, he wasn't riding in the elevator alone, he had his mother with him. This was something that obviously "bothered" people....but it was a non-issue, and they needed to mind their own business, because at that age, he hadn't been taught how to discern when and if it was appropriate to take direction from strangers. He had, however, been taught notto touch the button.
See, but you're one of them there responsible-type parents, who would agree that it is Not The Thing To Do to draw on other people and would want to know. There are the other types (and I think that this is the type most people are referring to here) that think I am being a big meanie for stifling their child's creativity. There are also those that want other adults to discipline their kids because they don't want to appear "mean." Seen that, too.
If your child, for example, had slid off (and they can be fast suckers...) and was about to put his hands into the punchbowl, I would say something and then follow it with "Let's go find your Mommy" (if I knew the child's Mom) or "Can you show me your Mommy?" (if I don't).
I don't think there's a single thing wrong with that. If you disagree, fine. But I am not fond of punch that had hands in it, child or adult.
I certainly wouldn't say anything about the button pushing unless his little fingers were ooooooooh so close and you were busy talking to the person next to you and didn't see. One of my favorite things to ask a little kid about to do something out of eyeshot from their parents (or below the radar) is "Are you sure you want to do that?" with a smile. Kids generally stop to think about it. The "thinking faces" that little kids have can be so cute.
That generally gets the attention of whomever's kid it is and they handle the situation accordingly. And this, too is only used in situations where the child's action would have "consequences" (drawing on furniture, putting their food under the sofa cushions (yup - seen it), pulling or touching hair, etc). Staring at me is not on that list.
I'm a "skin off my nose" kinda gal. If whatever it is that's going on is no skin off my nose, then I really don't care, even if it is irritating (hope that makes sense). I am not fond of kids staring at me over th booth (not fond of ANYONE doing that, really). But - it's no skin off my nose. That, and I can make some really good faces. If I can make a staring child giggle, it's a good night. But leaning over, trying to touch or grab - no.