Because the world is not static and the phrase "not a threat to our interests" is almost meaningless. They may not be a threat today, but before you know it they invade Kuwait, or Poland, or bomb Pearl Harbor, or fly airplanes into the World Trade Center and the Pentagon. Not my Japan or Germany, not my problem. Not hardly.
How will russia bomb pearl harbor or invade poland as long as the nuclear deterrent exists? Is there a Ukraine domino theory? Russia is a shell of its former self. Draining our resources picking every fight like Libya or picking completely unnecessary fights with a nuclear armed country are bigger threats to our long term security than Russia occupying parts of the world they have generally occupied.
That is my whole point. You have to ask "does this action build to anything?" In the case of Russia and the Ukraine, what would Russia taking the Ukraine build to? It does not affect america either way. Not every bad action in the world will affect us and not every bad action must be stopped. Intervening in matters that do not pertain to our interests will more likely hurt us than help us.
Regardless of what the Europeans want, we would be better off if Crimea, and soon the Ukraine, were democracies instead of being run by a dictator.
If making the Ukraine a pro western democracy causes war with Russia, we are not better off. If it causes Russia to stir up trouble elsewhere, we are not better off. We are better off finding ways to work with Russia against ISIS. Not every little country in the world needs to be a democracy to keep us safe.
How many wars were started by truly democratic(those elected absent of fraud) countries?
The War of 1812 (we started it to try and take Canada)
The Mexican American War (sort of. Mexico wasnt a true democracy and Mexico did start it. However, the US certainly wanted to fight it.)
The War Between the States.
The Spanish American War.
The Boer Wars.
Democracy has not changed the fact India and Pakistan hate each other and have fought over the last century.
Democracies are only good for us if the people elect pro US leaders. If a pro US dictator is overthrown and the people are very anti-american, we would be better off with the dictator.