This is not real life, it doesn't have to make complete sense if it is good for the game play. I don't understand how that per ship sensor limit is going to help any more than ship range limit, but the problem kinda exist in the GC2 and who knows about the GC3.
It does have to make some kind of sense, because having it work the way you describe is just going to leave a lot of people scratching their heads and going "WTF?"
If you want to attack colony spam, the way to do it is not to make everybody blind.