This article is from the Douglas Adams FAQ, by Nathan Hughes firstname.lastname@example.org with numerous contributions by others.
Why was Pink Floyd's music taken out of the radio series? Well, from the
"That's a very good question, and quite an instructive answer (I think!).
The BBC Clearing the Use of Seventies Rock Music in Comedy Science Fiction
Stories Department tried to clear the use of this snatch of music. They
didn't ask me about it and had no idea that I knew any of the band. In fact
there's no reason why they should ask me P if I had every detail of every
obscure copyright negotiation referred back to me it would be a full time
The BBC department in question would have been dealing with PF's lawyers,
and exactly the same applies. So we were all locked in a legal battle
without any of the principals having the remotest idea.
Another point is that I think that the actual piece was probably
copyrighted at least in part to Roger Waters, whom I don't know at all. In
fact, being a friend of Dave and Nick would probably have weighed against
me in that case..."
Anyway, to sum it up: Copyright Law. 'nuff said.