Actually, I think that Clear Channel was the first to do this several years ago and they took a lot of heat for it because they didn't clearly disclose that these were "national" contests. In fact, they promoted them with the illusion that they were local contests - cleverly, they never actually lied about it - and (IIRC) ended up in court over it. Now, such promotions must be announced as combined or 'national' contests.
CBS is actually pretty late in the game and, if it's limited to 2 Boston stations, they're not actually being that unreasonable. Clear Channel had contests that pitted listeners from several hundred stations against each other!