The band, along with Atlantic, own the rights to the lyrics. If you remember, Robert had made it an issue about sampling Jimmy's licks on his Tall Cool One song. He said that Atlantic could not sue him for using them, as it would be tough to prove that it wasn't Jimmy doing it in present day.
When asked in a BBC interview about the fortune he must be making in royalties off of Zeps music Plant said, I sold them years ago.
I don't have the interview, and honestly, do not know it to be fact. I am going by memory.