Yes, the Constitution says that Congressman may run criminal enterprises from the safe haven of their congressional offices, and there’s nothing anybody can do about it.
Which means, of course, that constituent letters were untouchable as were secret messages from the then Democrat majority leader in the House, or other Democrat caucus officers.
FBI didn't care ~ they read your mail anyway. It's still yours years after you send it to somebody you know.
Did they get a warrant on probable cause that your correspondence to this guy was part of a crime?
What you have here is a court that made the minimum decision possible to allow the cops to look at some of Jefferson's stuff while not also suggesting that the cops broke the law and should be punished (which is what I think the Supreme Court will say whenever it gets this or a similar case).