I'm actually not entirely sure why the official reasoning for the Cast Members now asking the guests to check their own restraints instead of the Cast doing it. I mean it doesn't matter, they're both safe practices, but being a theme park management major, it interests me.
One reason I could think of would be to prevent strain on the Cast Members backs from rides that are "lower" such as Barnstormer, Big Thunder Mountain. Some of the lapbars even have little "grab handles" specifically for the Cast to use, such as Everest and Space Mountain, but they're not used obviously.
Another reason (which is what I've heard is the official reason, but I don't believe it) would be to release the park from any liability. Since the guest is checking the restraint, and if the restraint isn't locked, it's now the guests fault since they didn't notify anyone. Yes, I know it's a dumb reason, but I've heard that. Don't yell at me and say it's a stupid reason, I know. I'm telling you now that I agree it's a stupid reason.
Anywho, all rides at Disney are safe if everyone meets the posted height requirement.