The answer is either...
They can.
or
That's the blackout rule.
I was initially thinking the blackout rule only applied to the same network. (i.e. if your team was blacked out at 1, on FOX you would not get another game at 1 on FOX.) However, now that I think about it, I'm not posititve you could get a game at 1 on CBS. I haven't had to worry about this since I was a tot.
Either way, the goal of the blackout rule is to try to be sure every seat has an ### in it, since the TV money is set ahead of time. Even blacking out other games in a similar time slot would sell a few more tickets, since a few people throughout the year would chose to go see a lousy team, as opposed to seeing no teams, especially if that lousy team played a good team.