Think that you are in full gear and ppl behind you don't really care on shooting/shashing you dead because they don't get any penalty. You lose all upgrades and have no money/evo left. Hmm.. You might see the dilemma.
Think that you're in the middle of a fight, you do care about your teammates because you know you'd get eaten in a sec without them, and you're as smart as to not left click when one of them is in front of you. Now one of them gets headshotted, he has about 5HP... And accidentally one of your bullets gets to him (hitboxes are usually bigger than the character model). Anyway, you'll get the flashy teamkill message.
But the real killer is the alien right ? And he'll probably get an evo for all the damage he inflicted. Nevertheless, great retribution system will take away your money.
This happens more often than your story.