I don't particularly agree or disagree with them, but they're making a horrible business decision for many reasons.
A) Steam already has a massive install base. It's likely that anyone who plays AAA PC games has either already used Steam or at least knows what steam is.
B) The overwhelmingly vast majority of D2D customers are also likely past Steam customers as well. Those that have avoided steam on purpose are not likely to purchase Modern Warfare 2 and even if bought are unlikely to become exclusive Steam customers.
C) D2D can absolutely 100% get away with selling keys to Modern Warfare 2, thus directing the customer to enter the key into steam and download MW2 via Steam only, saving on HUGE amounts of bandwidth.
D) Even if Modern Warfare 2 was offered as a download through D2D, they're still going to save on bandwidth. Most future reinstallations of the game will be directly from Steam, and smart users will realize that steam's 1 click install is easier/faster than d2d's download file, go through installation process, and then having to delete the download files manually. Having used both services many times, I can speak for certain that d2d's available download bandwidth is weak relative to Steam. I have never gotten over 750 kbps on d2d, and it's not uncommon for me to hit 1.8 - 2.0 mbps on steam.
E) D2D sells other steam-required games. Why did they sell those games, not MW2, and continue to sell the other games?
Steam hands down is currently a much better store/service/delivery method than any other digital store. Developers are taking advantage of steam's free tools and automated services which make their job easier and more seamless for the customer. D2D does not offer anything like this. Instead of complaining about how developers choose to build their games, d2d would be better off trying to improve it's own service. In the meantime, d2d is losing out on quite a few sales.