It's interesting that the non-Cobert music made it into the DVDs, considering that there are Muzak versions of Beatles tunes in a couple of episodes. I'm guessing that because the show didn't include actual Beatles recordings, they might have had only to pay a standard royalty.
Regarding WKRP, that show was made in the late 1970s and early 1980s, when the home video market was still relatively new, with a focus on rentals rather than sales. New releases were priced at $90 to $100, in order to force video stores to pay significantly for rental stock, and then eventually, some titles would be re-released at a sell-through price. Home video releases weren't a given the way they are now, and when licensing music for use on TV and in films, producers tended not to negotiate beyond broadcast use because home video releases (especially of TV shows) weren't on anyone's radar. So, when Fox wanted to release WKRP on DVD, they had to re-negotiate the use of every piece of music. The unfortunate thing about this is that the owners of those songs can pretty much ask for any amount of money, often making it cost-ineffective for the producers to use the original songs. (And I could be wrong, but I think that "Stairway to Heaven" is simply not available to anyone anymore.)
An interesting exception is Peggy Lee and the film Lady and the Tramp. She wrote and performed songs for the film and had it in her contract that she would receive royalties from all avenues of distribution. This, of course, was long before home video existed, but when the film came out on VHS for the first time, her estate successfully sued Disney for back royalties because the courts ruled that home video, although not in existence at the time, was definitely an avenue of distribution.
I imagine that today, producers take into account the home video market, as well as the Internet and other forms of distribution, and negotiate the rights to use songs accordingly, thereby avoiding the need to re-negotiate.
