A valid point Mark, and I would never advocate using a reversed rotation chuck without a locking screw, but in actual practice the worst that normally happens is that the chuck and whatever it is holding falls onto the lathe bed or the floor if the headstock is turned.
Bad enough if fingers are in the way between work and the tool rest as it jacks out but most unlikely to do other than drop.
By its very nature the chuck has to stop turning at speed to unwind.
I've seen it happen on metal lathes where it makes a mess of the tooling as it jacks out but it is stationary whist it is creating havoc.