Right - I'm guessing these folks didn't have buddies.Try it once - it's part of open water training as I understand it. Turn your air off, and breathe thru your reg, how many breathes till it starts to get some resistance (I suspect different regs "feel" different), you got one breath left at best.
I see many people turn air on to check for pressure then turn off for the boat ride out - they don't do a precheck or a buddy check - it's that simple.
Complacency
Pre check and buddy checks are taught in OW, at virtually every step of advanced training, it's re enforced, it shouldn't happen pure and simple but it does cause we are human.
Advanced training helps one deal with stress, helps one avoid panic - we are taught from the beginning solutions to virtually all problems that can occur but panic wipes that training out and can result in an unfortunate tragedy.
I didn't have that training for OW. Take mask off, yes, but not air off. However, I'm starting with a volunteer dive team at an aquarium, and they said after a few weeks they put you through a "panic test" underwater. I'm interested to see what my reaction to things are!