The answer, as told to Peter Salus by Dennis Ritchie:

"The ; was utilitarian. During most of the early '70s the most popular terminal was the Teletype model 37. The sequence <esc>; put it into full-duplex mode so the terminal didn't print characters locally, but let the system echo them. So this sequence was put into the greeting message. Of course it didn't print when you used that terminal, but other terminals that appeared later didn't understand the message and so printed the ;."

Peter H. Salus, A Quarter Century of UNIX, page 69.