Some of those may have been not locked properly and left on the hydraulics over night and drained down. The Camaro looks to me like it was gently placed there, no broken glass on the floor and the hood most likely would have been crunched if it had fallen. The four post ones I bet were the same. If you don't lock all four locks, and confirm it, you will get that result when you come back the next day.
As far as these idiots who are too lazy to make sure the car is secure that's a different story. I've seen guys just kick the arms under the car and press the button. When I put anything on my lifts, it takes easily 10 minutes. First you end up repositioning the vehicle, then you have to get the arms in the right spots to line up with frame pieces, not hit anything important, run the arms up close, look underneath, run them up a little closer, adjust said arms again, some times lower the arms and/or re-reposition the vehicle, run them up again, finally get them where you want, lift the vehicle a few inches, try to push it off. Oh and I forgot all the cursing! What I can tell you is I'm very comfortable when under there. When I do lower it on the locks, I make sure there is no residual hydraulic pressure.