So how do you effectively enforce that? Should the owner have cameras in the hallway? Should the tenants be subject to random checks? some balance has to be found to keep both sides happy.
The other question is why are tenants going through the trouble and risks of subletting. And this guy knows why:
http://moodyeyeview.files.wordpress.com/2011/08/toodamnhigh....
You might notice Airbnb has virtually no presence (relatively) in countries where the rent is lower, especially in relation to people's income.