With the caveat that I don't know if there was an exception for a New York to Los Angeles service, Pan American didn't have domestic routes prior to purchasing National (the vanished Miami one, not the vanished Las Vegas one) in 1980. Some make a case that the added debt of that takeover, right at the start of airline deregulation, was the first nail in Pan Am's coffin even though the Clippers would keep flying for another decade and more. Now, that lack of domestic service wouldn't mean that -- especially considering that in the 1970s and before there weren't any airliners which could cross the Pacific nonstop -- refueling stops in Honolulu didn't take place, and that tickets weren't sold to those people who wanted to go to Hawaii rather than fly all the way to Tokyo or Sydney.

I don't think there was any legal restriction that kept Pan Am out of serving U.S. domestic city pairs but simply the fact that the airline had gotten its start as an international carrier (the first Pan Am flight was a mail run between Key West and Havana) and the airline's management simply felt that specializing in long-haul international flight was where the company's best interests lay.