Yes because some (most of the ones worth paying for) of the games require some sort of server. So every new player in a game is not free for EA. Giving away a game means they will lose money on this person if they play online.
Say a total of 5,000 downloads are from this "free" coupon for any specific game title. Would $500 or $5000 sound closers to the actually hosting/support cost that the publisher has.
It's pretty much a win for EA which is why I expect they let it continue so long.