I actually just truly don't understand...

When I go places with friends and my son, either out to eat or to their house for a party/BBQ, I know that I am responsible for watching my kid (he is 3). I try to let him play on his own as much as possible, but also make sure that he is not being a bully or pulling the cat's tail, etc. Yet, when other people (aquaintances, coworkers)come to my house, or go out to dinner with us and their kids, they just kind of dump them in a chair or somewhere close by and act like they don't exist and let them do whatever they want...I end up watching them cuz the parents don't seem to care...what is up with that?? Even when the child cries or gets hurt, they just ignore them (their kids are around the same ages as mine). Why do people not watch their kids at all? Some of them even ask me later if their child behaved (which they didn't), as if I'm the babysitter or something... Or am I the only one who has had this experience?