This is really, really stupid...3 reasons
Game was sold at a premium price and very successful. They could at least give their loyal customers a little something extra.
15$ is half the price of some AAA games like Mass Effect 2. It's just too much for a DLC like this. Especially for MW2 that was sold at premium.
Thirdly and most importantly, it's MP maps which means asking a price for it will splinter their entire online community since not everyone will buy it.