Generally illegal contracts are unenforceable. Ostensibly, the law is not intended to help secure ill gotten gains.

This is a weird one because pot isn't very illegal. If the landlord had gone to the state court, this wouldn't be an issue. At the state level it is legal to possess, buy and sell weed. Dispensaries are just another tenant.

However, the landlord went to federal court. In federal court, it is NOT legal to possess, buy or sell weed. In fact, it is illegal to even rent to "dealers".

Which brings up the question. If the federal judge rules that the contract is enforceable then he's ruling that it is legal.