At one point in time, Medial General in Tampa owned The (now defunct) Tampa Tribune, WFLA (AM), WFLA-FM and WFLA-TV. MG would eventually divest its radio products in the market to another company, retaining the newspaper and the TV station.
From what I have always been told by people who were at the time, is that the new owners of the radio stations had the "rights" to the call letters, and they (and subsequent owners all the way to iHeartMedia) allow Media General to use them for WFLA-TV. For what price I don't know. I've heard rumors ranging from a cash deal to simply providing the radio stations with weather forecasts voiced by the TV's meteorologists.
(As an aside, the FM station would have its calls changed to WFLZ. Clear Channel would eventually place the WFLA-FM calls on a station in Tallahassee, and use the "WFLA" brand on stations in Florida whose calls were "WFLF" and "WFLF-FM." Furthering my belief in this case that the owners of the original WFLA (AM) hold the rights to the calls.)