Allergy specialists refuses to let me self pay.

I started out seeing an allergy specialist using Aetna insurance. I was approved for a years long dose of a personalized allergy injections through the insurance. I started receiving injections on a weekly basis that my insurance paid for. About half way through my injection schedule, I lost my Aetna insurance and had to go on state insurance due to the death of my husband. My allergist specialist claims he does not accept my insurance and so I paid for my office visits with the nurse to give me injections, out of pocket. I had to meet with the Provider to get a different medication so I paid cash for that visit too. As I was leaving the Provider told me that he wouldn’t be able to accept me as a patient any longer since I’m paying cash. He said, “I’ll have the office look into it because I think there’s a legal reason I can’t take cash” and said he would have someone call me. Well, my guess is that he really does accept my insurance but doesn’t want to take patients since the provider compensation isn’t too great. Legally he has to bill my insurance if he’s contracted, right? But since they are telling me he doesn’t accept that insurance why can’t I be cash pay? This is in Washington state if that matters..