Solzhenitsyn wrote in The Gulag Archipelago that part of the problem with proving one's innocence in the Soviet legal system (apart from the KGB torture rooms) was the fact that the Soviet legal code was never released to the general public until after the Soviet Union crumbled in 1991.
Consider:
"You're under arrest!"
"For what?"
"You broke the law!"
"Which law?"
"That's classified."
I agree with the need to check ID's, especially at travel terminals, but in a free society the law should always be clear and accessable.
The applicable laws have been posted already on the thread. There are no classified laws related to public transportation, and only anarcho-sheeple would assume otherwise.