I've never had an issue with companies offering an incentive to buy new, but what they are doing isn't offering an incentive. They're not allowing you to play the game as it was meant.
First day DLC, codes for extra maps, etc. are all fine and good in my book...they leave it up to the consumer as to whether or not that stuff is worth it to them. If it is, buy new. If not, whatever.
Mortal Kombat, at it's core, is a multiplayer game. Releasing a game and NOT ALLOWING PEOPLE ACCESS TO THE MULTIPLAYER is a dick move. Offering multiplayer for people who buy new is not an incentive, it's an imperative. Essentially, they made sure that if you purchased a copy any way other than brand fucking new, you were only purchasing half of a game. They would have been better off by offering the incentive of a code for a couple extra characters, different fatalities, a costume pack, something like that. You know, CONTENT? Multiplayer is not "extra content", multiplayer is the heart of the game itself.
I love Mortal Kombat, and have fond memories of going to the arcade with friends to play the very first MK on the arcade machine. That said, I can't wait to see the sales of this game tank.