Far too complicated for LE to enforce. They would have to know the laws of their own state, which is already a challenge for some, it seems, plus the laws of the other 50 states. With a rule like this, you would probably be arrested and taken to the station. There, you would sit for a few hours until someone (a judge?) assessed whether you've violated the laws of your own state. Doesn't sound appealing to me.
This would be most useful in cases of transporting your firearm and firearm related items through restrictive states. But, of course, we already have that protection through FOPA.