Abandoned three-letter calls are all retired just because the FCC doesn't issue them anymore for new stations. In the case of WBZ-FM, WLS-FM, WWL-FM and others, those are grandfathered in, and it's merely the station owner picking up the similar FM calls for a local sister station on another band with those calls. That's allowed.
In the case of KHJ, with the KKHJ calls on a Spanish-language station, the TOH ID was a bit awkward (as "KK" in Spanish is pronounced "Kah Kah", and I won't tell you what that word means in that language). The station's owner asked to bring back the well-known KHJ calls. Some other situations (like WJW) are case-by-case. What likely happened was the TV station's owner struck a deal to bring back the calls when the AM station decided to abandon them.
The FCC does not 'retire' call letters. At all. Not even for historic reasons. There are way too many historic call signs out there to do that. Every market seemingly has a historic station. If, say, Disney and Citadel both decided to drop the WABC calls altogether to go with something else (ain't gonna happen, but let's be hypothetical), any station could pick up the abandoned calls.
And the only reason WNBC won't ever be back on AM is because the call sign belongs to WNBC-TV, and NBC/Comcast no longer owns radio stations, and they certainly won't let another station use them.
If a call sign is currently being used on any AM, FM or TV outlet, the same calls can only be used on another broadcast band by the present owner, or by permission by another party. And 3-letter calls are no longer issued for new stations, except for the scenario mentioned earlier.