Some kids have absolutely no boundaries, and I was one of them! I was one of the most terrifying types of kids to people who don't like being touched; I was a hugger, and nothing could stop me from going up to absolute strangers and wrapping my gangly arms around them. Somebody really should've told me to ask for permission before hugging every adult in my midst, but even if they did, I probably wouldn't have listened.
There are so many kids, especially those in the elementary age group, who will do whatever they want and chance all consequences. It doesn't matter if they've gotten ten lectures about how morally reprehensible their behavior is, they're still going to test it. One teacher who has done her best to instill in her kindergarteners that it's not okay to drink from other people's drinks found that her lessons have not stuck with at least one of her students who needs her caffeine.