The same was true when CBS moved WBCN to HD-2 and the Free Form station that was on WZLX - HD3. To me it makes no sense that Beasley now parks the calls in Florida after they shut down the Charlotte station where CBS originally parked the calls. They don't run the HD2 station anymore (changing it to Hubcast shortly after they assumed control of WBZ-FM). And, what was WBCN would be suited to be resurrected with a modern rock format (if it was ever to come back), not a direct competitor to WBOS.
The "C" in WBCN might make the call useful to some station outside the Boston market that Beasley wants to flip to country, so why not hold onto the call instead of having to look for one that isn't in use later? Big Country Naples? Bangor's Country, New? (Just examples, I'm not going to look up all of Beasley's markets for the sake of this one post.)