For criminal concerns regarding retroactive ITAR additions, yes. However, significant civil financial penalties if congress so wished could still be constitutional as the ex post facto clause has been held to apply exclusively to criminal matters starting in Calder v. Bull [1].
History is littered with unconstitutional, enforced laws, as well. Watched a lot of Ken Burns docs this weekend while sick. “The West” has quite a few examples.
There are a lot of things in the US Constitution. But the Supreme Court is the final arbiter, and they're moving closer and closer to "whatever you say, big daddy."
