The exception to this is where one State, by way of policy, does not accept the judgments of another state; Types of marriages comes immediately to mind.
Actually ... congress had to pass a law to allow this (or at least to keep it from being defeated in court challenges). But that's congress's constitutional authority, to control how these things must be accepted/can be rejected between states. Without this law, states trying to ignore gay marriage certificates from other states almost certainly would have been defeated in court.
story from 2004
Personally ... I still think if Congress were to pass a reciprocity law (they won't, at least not at this time) I wouldn't be suprised if it would stand up in court. Not everyone involved in law making is an idiot, and if such a law is passed it will be tried in the courts/resolved in the courts. Not by internet posters. And there is an argument to be made, and only whether it is a winning argument or not is yet to be determined. Remember that federal assault weapons ban? That was never defeated in court. The ban on machine guns after 198x has not been defeated in court and has been the law for over 20 years.
And I'm in favor of a reciprociy law. You should be too. Why?
It's one more law that works for us, one more "privilege" given the American people that anti-gunners have to fight against. Because the theory that this concept is bad because "what the government can give, the government can give away" is just not well thought out or rational.
If you think for one moment that if Boxer, Feinstein and Schumer find themselves with an anti-gun majority, they're going to talk about outlawing CCW in all states (how they would do this constutionally I don't know -- not part of the debate) and then say ... "Oh, but we can't do that because we don't currently have a law giving people reciprocity between states so there's no way we can take it away ..." Well, that's just naive and funny.
The fact is, if we did have reciprocity between states, the gun banners have one more hurdle to deal with. Now they not only have to take away a privilege granted by 40 states, they have to take away a privilige given nationwide. American's don't like priviliges taken away.
A law for nationwide CCW reciprocity, if it would pass constitutional muster, would be NOTHING but good for all of us.