Most of the keys are obtained illegally (stolen accounts and/or credit cards) so eventually the money gets taken back. So not only was the game stolen but the indie has to go through processing the takeback which costs them money on top of it.
And since the takeback issue can occur the person purchasing could lose their game without even realizing it and then complain to the devs when it wasn't even their fault.
You're basically double-dipping and ensuring that actual costs are involved.
edit: brain fuzzy. Chargeback is the word.