J_SI agree with whoever just mentioned laziness. That's how it is for me, I just don't care enough to put it down. It's also annoying how you have to duck your head in, and everyone has to scoot to the right spot to make sure your ski pants dont get caught underneath the little peg. It just causes a lot of problems.
The truth is though, ski chairs are slanted backwards so that gravity is literally pulling you into you seat. If you have every intent to stay on the chair, you will stay on the chair. It's not like riding a mechanical bull or anything.
I'm not sure what the exact scenario was for the first kid in PA that fell, but for the more recent one, didn't they just jump off on purpose? That's the thing that leads me to believe the lack of a bar isn't an issue. For little kids, that's a whole other story, but for this, there's not much you can do to keep people from making unsafe choices other than to chain them to the chair with a padlock.
They definitely are better and safer than if they didn't exist, but they aren't even close to perfect. It's still possible to get hurt with them. I believe that 90 percent of what they actually do is just comfort the passengers, whether it's peace of mind or just a way to rest your feet. If someone wanted to mess around on the chair and risk falling, the safety bar won't do anything to protect them. Unless it's super windy or a sketchy lift, I feel safe enough without it, so my laziness takes over and I go without.