Is there some law that limits the flight of armed transports and/or starfighters on Core planets?

Well, starship captains need Imperial-approved Core Worlds Travel Clearance documents in order to approach, so that acts as a blanket statement regarding any traffic. Even if they had it, there's almost certainly a ban in place for starfighters. However, Imperial forces don't need little things like laws since the Empire is effectively under martial law.

Ok, and on Outer Rim Worlds it's easy to fly the surface with Starfighters without getting to muuch problems?

As a GM this is where you can decide. On a core world it's pretty locked down, but if you want to let your group go blasting off to parts unknown on say, Ryloth, then that would work.

My group wanted to take their ship and strafe the enemy compound they were supposed to attack. I told them they could do that, but they would need clearance to leave and the local garrison might chase after them if they just blasted out of the spaceport and took off. They decided not to risk an atmospheric ship battle and then attacking a potentially well defended installation and went in on foot (which is what I was hoping they would do).

LukeZZ said:

I'd say it depends on the world. there are some Outer Rim worlds without much Imperial interference, and some that are locked down pretty harshly. Mon Calamari, for example, is occupied by Imperial forces in an attempt to control the shipyards there. Tatooine, however, seems to be a place where you could pretty reasonably come and go in a fighter or a fairly heavily armed transport (dual quad laser cannon turrets are nothing to sneeze at).