Owners do not have an unlimited right to decide whom to rent to. They can decide to rent to no one, but once they start offering to rent they must follow the law.
When you're SELLING a five star product, and you choose only to SELL it to [people who you look upon favorably] then you're discriminating.
Someone looks at one star homes (for example) on AirBnB, and five star. Chooses a five star. They haven't discriminated against any protected class in choosing the five star home. The homeowner absolutely can by refusing.