I thought the same, different market, -FM didn’t need permission from owner of the (AM) call, but now I’m thinking Jeremy is right? Or it is because of future aspirations for both calls.

BTW there is never an -AM suffix, only -FM or -TV, if the suffix-less call exists. It will just be WTBC and WTBC-FM, often written as WTBC (AM) and WTBC-FM. I remembered this from the short period of time when WXRT had to change call to WXRT-FM while 1160 AM became WXRT. When 1160 flipped to WYLL, WXRT-FM changed call back to WXRT. WLUP went through the same gyrations.