They can’t police it, obviously, and since the whole of the league’s area is in either Spinal Tap 1 or 2, the risks are minimal. I think it’s to discourage travel generally, especially where shared cars have been identified as an influence on the spread of the jandies in the processing plants in Buchan, and the cow and Peppa factory in Portlethun.
If some individual from Grantown decides to drive to Wick, I doubt that some Harmjobsworth’s going to tell him or her that their tenner’s not welcome.