No crime committed for just "talking" about running a re-broadcast (and getting the publicity, which KCAA did.) No law against saying you'd do something.
CBS owns the copyright (and protection provided) for the program, by contract with Imus. Westwood One is the mere program distributor.
The "intellectual rights" to the program broadcast belong to CBS Radio for every program and includes, usually, in an affiliate agreement, the right to (or not) rebroadcast outside of a contracted period of time.
They waited, also, to make an effort to put teeth into the cost of lawyers, etc. and to set the stage for an out of court settlement. Running the program (evidenciary) as KCAA did for five days at $150,000 a clip for federal copyright infringement is pretty teethy.
It will be moreso if the station defies the apparent C&D next week and, especially, a TRO if ordered by a court.
Westwood One had every right under the contract to cancel the distribution agreement and to be held harmless. CBS cancelled the Imus program.
From contracts of their's I've seen, expecially in a "for cause" cancellation, they aren't required to substitute another program or conpensate for the cancellation.