In many places, a landlord can't just decide to raise the rent. When I signed the contract for my current apartment, the landlord needed me to sign an official form they had to send to the city. The form informed me of the rent the previous occupant paid and stated the applicable laws that regulated by how much the rent could be raised (based on inflation, changes in interest rates etc.). What I don't know, but assume, is that it's easy to fight for the rent to be lowered to the legal limits even after accepting signing a contract with rent above the legal limits.