Why? Like Clinton said, "Because we can."
Say what you want about the motivations of the US administration attacking Iraq. A lot of Americans will agree with you on that. President Bush was just recently reelected by the smallest margin ever for an incumbant, so don't assume that all Americans agree with him.
But how does McDonalds in Moscow translate into evidence of some nefarious plot? It's called international trade. Americans buy a lot of stuff from your country. In fact, we set a record trade deficet last year. MickyD's wasn't installed in Moscow by a military invasion. Some Muscovite decided his fellow citizens would enjoy eating greasy cheeseburgers and soggy french fries enough to pay money for the privilage. McDonalds being in business to make money simply obliged them. No one is forcing people to eat there.
By the way, you have got Shi'ite and Sunni reversed in your post. The Sunnis are the 20% minority that is presumed to be loyal to Saddam. In reality most of them probably saw he was a bastard too, but at this point they are facing the very real prospect of being on the wrong end of a round of ethnic cleansing, so it would be natural that they might long for the "good ole days".