First off, using your logic it's perfectly morally okay to taze a cow that's about to be slaughtered for no reason just because it's going to be food... Which is of course a ridiculous statement. Yes, the mouse is about to suffer, but we owe it to the feeders we use to respect them enough not to make that suffering any longer or worse than it has to be. Additionally, a properly bred feeder's response to danger is to freeze, not attack. The freeze instinct has been selectively bred for, so no they do not go into 'fight mode' the second they are in a snake tank.
Secondly, there is a safety component to not 'thumping' or stunning the mice before feeding them. This automatically puts them into a defensive state of mind (after all you just attacked them, and you're not as good at immobilizing a mouse as your snake) and makes them more likely to bite. Additionally, they are not moving normally after being thumped so the snake has a harder time grabbing them properly, again making a bite more likely.
If you had a bite occur after doing this, that's why. If you had a bite that did any major damage (I'm not talking about a minor scratch) then it's because you were not properly supervising the feeding and were not ready with a utensil to put in the mouse's mouth to keep it from repeatedly biting long enough to do damage.
The advice you just gave is dangerous, cruel, and just plain incorrect. I'd suggest learning a little more about how to responsibly live-feed.