Why do they do it? The gates are literally there to contain small children from wandering off. Yes we all watch our children, but it would be made easier and safer if people closed gates behind them.
I'm a childminder and have just spent the morning at a trampoline park, big ones were off jumping and I was playing with the little ones in the under 5s bit. I must have closed the gate 20 plus times! It wasn't even my children trying to get out but I was mindful that the main door, the cafe with hot drinks and the big steps to the trampolines were all close by and not suitable for unaccompanied wandering little ones.