Landlords are under no obligation to call the feds when a tenant is growing marijuana.
During the dark years of Dick Cheney, landlords had to worry about their property being seized by the D.E.A. for property drug violations (and, worse than countries like China and Iran…. no charges are necessary for the DEA to just take your property away.)
Obama has put a stop to that crap, and is using the D.E.A. a lot more carefully and thoughtfully than President Cheney did. The issue of medical marijuana has already gone through the federal courts, and some aspects are not decided yet. But we do know for sure that landlords and local police are under zero obligation to nark on citizens to the DEA.
Landlords in California, Colorado, and Michigan are free to do as they please, on this particular issue, only having to worry about laws and liabilities that apply to all businesses in general.