It isn't necessarily. It depends on whether or not this movement leads to real liberation for the Egyptian people. We could just end up with military rule and a different dictator.
The biggest fear for the United States is that some form of anti-American, Islamist government takes over. This could happen in some form of an Iranian style revolution or possibly through the democratic process playing itself out.
Personally, I think that an American supported dictator like Mubarak was bad for American security. Sure, he may have maintained order and kept extremists from controlling the government. The problem what that he angered many of his own people, and the United States was somewhat blamed for the situation. In the end, encouraging the Egyptian people to shape their own destiny, even if in the short term they choose a government not completely to our liking, may be the best thing for American security. Whatever the case, I don't think that the United States is in much of a position to control these movements that are spreading like wildfire throughout North Africa and the Middle East. Here are some more thoughts: