They are BRANDS, and that was covered in the Audacy agreement. They can also be trademarked, if that process is done. That's why the Audacy agreement doesn't specify AM or FM. The part CBS controls is the brand of WBZ. But the assignment of call letters is done by the FCC and they control who gets WBZ-FM.
Sure, iHeart could "buy" the use of the FM call letters if it meant something. In Seattle, they controlled the KJR-AM/FM call letters. When KJR AM decided to simulcast on FM, they simply transferred the FM calls to that station. Simple. However, Cox didn't do that in Atlanta with WSB, even though they could. Audacy didn't do that with WBBM in Chicago.
But still, as we've said, there is no need to blow up an FM for a WBZ simulcast. It won't make more money. It won't help demographics. And it won't improve the product. The best thing iHeart can do with WBZ is better merchandise the content digitally. That's where the younger audiences are, and that's how they can make more money. iHeart doesn't own an FM that they're willing to blow up, so it won't happen. Talking about call letters is putting the cart before the horse.