Because the part of their operation that presumably is ROI-positive (the FM transmitter) is technically a rebroadcast of whatever they are transmitting on AM. I

Because the part of their operation that presumably is ROI-positive (the FM transmitter) is technically a rebroadcast of whatever they are transmitting on AM. If the quality of the AM signal is actually so bad that nobody can receive it, then they will forfeit their FM license too.