(two girls) Being upset in your familiar comfortable bed is a very different thing than accidentally trapping yourself in a tiny strange dark box and physically struggling to get out. Very different chemical things happening in your brain in those two situations.

Sure, I'm not saying parents have to come running every time or anything. The point is that letting a kid cry until he goes back to sleep is consistent. Locking your kid in a box to see what happens is not. How will the kid know next time he gets stuck, that it's not just another test? And since the kids are still learning how to communicate, adding a continuing doubt that maybe their parents understand them fine but have chosen to ignore them isn't going to help.