Irresponsible owner is only half of it, feeding live is never safe. Simply put, in the wild if the snake isn't immediately hungry, he moves on. Both animals confined in a small area, they're given no choice. Usually it is the rodent that suffers, but not always.
Here's a pic of a little kingsnake I got in that was with fuzzy mouse for about 5 minutes, watched the whole time. The owners saw the mouse doing something weird and took him out, but didn't even realize the snake had been bitten until the wounds started to get infected and swell a day or two later. In three days he looked like this:
All it takes is one bite to be fatal to your animal, or if not fatal - painful, disgusting and scarring. A bite can happen in a split second, before you could ever respond even if you were standing right there watching. I just think the risk is too great for my pets.