I look at it this way: housing is a basic human necessity. Structuring it as a standard business is a detriment to society. Sure, being a landlord is a business, but the whole "maximize profits" thing and "charge what the market will bear" thing should not be allowed for housing.

And the free market shouldn't apply to private property because...?

Free market doesn’t mean no regulations / rules / laws / protections. Of course all rentals are on a free market.