This doesn't need to be a legal issue. It's simply a rule of the building. People violating it are violating their contract with their building just like when they violate any other rule.
> Airbnb decreases the supply of housing and makes it more expensive for residents.
Long term it should not decrease the supply of housing. Visitors are either going to stay in airbnbs, with friends, or at hotels. If visitors stay in Airbnbs, then there is less demand for hotels space and more space for residential properties. Demand for hotels also decreases residential supply.
Ultimately the issue of Airbnb on housing supply is a small one. Instead, question why so many neighborhoods have height restrictions and restrictions on building anything new. Most people supporting these laws are landowners and will admit that they are trying to protect the value of their property (i.e. keep housing prices high).