The CC companies pay money for them (I have no idea how much). AMEX, in helping Delta during bankruptcy, bought millions of dollars of miles in advance to give DL the cash at the time.

Just be happy that AA hasn't started awarding EQMs for credit card purchases. Go over to the DL forum and see the complaints about the new Delta Reserve AMEX (which in addition to EQMs gives users upgrade priority over other fliers of same FF tier).

I personally don't mind credit cards giving miles out. Might as well, since that is what makes programs profitable for the airlines. It's not quite fair to flyers, but if the programs weren't profitable, they'd be eliminated. Flyer-based miles may not be particularly profitable.

Edited to add: looks like I was beaten to the answer!