Who is responsible to relocate cable TV line?
Like Chris says, you'll likely have to pay for this. If the line had become damaged through no fault of your own, they would pay to run a new line, and at that point you might get them to run it a different way.

That said, cable lines tend to be buried fairly shallow. You can dig it up yourself and rebury it a short distance away. Just be careful when digging it up since they are also easy to damage and they'd likely charge you for the repair. And when you rebury the line, try to get it deep enough so it can't be hit with an aerator or other routine yard maintenance.