Remember when the Dixie Chicks went off on the US when said they were ashamed of the this country while on a tour in Germany? I told my brother that I would never buy any of their music. He responded that it was "freedom of speech". I responded that they were free to say whatever the hell they wanted while I was free to reject their product.
This is kind of what Sterling is facing. He was free to say what he did, and he is free to say it again. But there are consequences nonetheless. Consumers are free to reject buying game tickets, sponsors are free to walk, and the league must protect its brand. I am sure there are stipulations in his agreement with the league. There will be consequences.
Sterling owns an NBA franchise, the brand is the NBA.
Like any other franchise, anything that the franchisee does which reflects badly or hurts the franchisor's brand, can result in the franchisee losing their right to the use of the brand. McDonald's is incredibly zealous about their brand image, and franchisees have to walk a very rigid line to hold on to those franchise rights that they paid for.
I haven't seen what the specifics of the agreements between the NBA and it;s franchisees is, but I I'm willing to bet you that they can in fact force him out if they want to.