Aetna processing as if secondary to Medicare

Wondering if anyone has ever come up against this.  Client has Part A Medicare, no Part B.  Client was in IOP and Aetna has processed claims as if Medicare is primary and adjusting off most of the charge. Aetna is telling us this is a penalty since the client declined Part B Medicare. I don't see how they can do this. They are penalizing us, not the member.

Thanks