And what would a couple of local kids be doing out at 1 am in the morning in the vicinity of the local prison? Not exactly visiting hours!
And when the foreigners usually aren't drunk or partying, drunk or drugged" when they actually rent the motor bikes, how is the renter supposed to know that they will be "partying, drunk or drugged" later? (Of course, young Thais don't party, get drunk or drugged and drive their motor bikes at high speed on the roads in the early hours of the morning - do they?) However, you have solved the problem by doing away with the motor bike rental business altogether ("curtailing") Maybe you mean adapting the rental business to rent only escooters to foreigners? Is that not a bit racist? Why not charge them up to 5 times what you would charge a Thai - they do that in public attractions don't they?
Your post has more holes in it than a honeycomb!