Well no one has mentioned the number1 reason, probably for political correctness, but "control the high ground". Even though treaties have declared space a weapon-free zone, whoever controls space controls the planet. Just like control of the skies was the source of victory in WW2, control of space will be the source of victory in the next big war, if there is one.

How many times have you heard on the news about a space-launch which was classified? It's usually just a quick one-line statement from the news-anchor, that a rocket was launched today carrying a claasified payload. The next "big-war" will happen fast (if it happens). In the meantime, it's the best place to spy from.