I can't believe that people don't see why Americans consider Russia a fair target. For all you younger people out there, the Cold War only ended 20 years ago. Many people can still remember a time where America and Russia were literally a bad word away from turning each other in cinder (and taking most of the world with them). And that time period lasted for over 40 years, and was the impetus for both the Korean and Vietnam war, as well as a myriad of smaller conflicts. You don't just get over something like that.
Plus, its not like we are the only country in the world with some dated, somewhat silly rivalries. Britain/France? China/Japan? Germany/The Rest of Europe? Wars tend to do that. Besides, no one here (except for your standard Tea Party whackos) actually hates Russians. At worst, we see them as "Those guys who live half a world away who we fought for a long time", which really can sum up a good chunk of the Europe as well. Most Americans, from my experience, see Russians simply as white people with funny accents. If anything, that just makes them an easier target because its hard to call "racism" when you have white people shooting white people. MW2 already played its brown people card with the Brazilians AND the Unspecifiedistanians, so I guess they wanted to go with a more traditional enemy.
As far as the MW universe goes, Russia actually makes perfect sense. They have a strong, developed military, and a militant Ultra-nationalist faction that is in control of the government. Combine that with the joint SAS/Marine task force that killed their leader just a few months before the start of the game....it makes sense they'd want revenge. Makarov's attack on the airport and Pvt. Allen's dead body just gave them a valid excuse for war.