That all come about from the issue of a series being pre full blown digital revolution.
See when Happy Days and WKRP and Northern Exposure first aired the issue of selling Video Tapes etc. of TV series was not a fully developed business model for the networks. So when music rights were negotiated they only worried about "Performance Rights" and using a Mechanical Production inside another Mechanical Production (using a song as part of a movie or TV series Video Tape.) was still not used much and most that were were usually theme songs that the show makers held all the rights to.
Now Performance rights are very straight forward. As long as I pay the fee to a performing rights organization OR directly to the people who hold the performance rights in case the are unaffiliated with a PRO then I can use it for a public performance like a radio Broadcast or a TV show broadcast etc. At least that was how it worked before we got DVR and On Demand. Now the Mechanical Rights must be cleared first.
But see Mechanical Rights are much different. If I hold the mechanical rights to a piece I can stop you from using it in a new Mechanical piece. This was set in stone in the "Sampling law suits"
Now you cannot use a nanosecond of a recording of previous mechanical piece without clearing it first.
So the reason those older shows don't have the mucis in them is either because they don't want it in there OR the people putting out the DVDs etc of the show are not willing to pay the huge fees most of those mechanical Rights holders want
Thanks. I was aware of issues of that nature and certainly I do not dispute the things you discuss. Forgive me, I tend to be something of a TV historian/purist and I prefer shows to be sold or broadcasted in their original format. For example, Paramount Pictures should have (while he was still alive) found a way to have reproduced “Star Trek-The Cage” with Malachi Throne’s original voice as the Keeper when a second colour negative was discovered (Gene Roddenberry gave his one and only full original, colour copy to NBC to chop up to produce “The Menagerie” two parter in which Throne’s voice was changed to the familiar sound), so Trek fans could see “The Cage” as it originally looked and sounded to the first test audiences.