[quote=@UltikanaRe]
You misread the post. Each circle on the map increases by 50,000 km in diameter. I also mentioned that was the stations immediate radar sphere. That means that it's own scanners could detect anything within that bubble.
[/quote]

Your wording says otherwise, but alright. 

Even so, why does a single nation of Terra have that much space-based infrastructure yet it hasn't colonized every planet in the system by now?

Realistically, your nation wouldn't permit any other powers to possess orbital assets. You'd just shoot down everything that tried to leave the surface of the world with your miniature Death Stars or space stations so that your faction could maintain space superiority and maintain hegemonic rule over the entire world.