Let the feds try to enforce. If Missouri stands by its word, then the state police will back the state and not help the feds. Are people getting arrested in MA for marijuana possession?
The feds have already reluctantly let a lot of that slip because the scope of it is such that its a difficult thing for them to do. Particularly when the pot isn't crossing
state lines, etc, last thing they want to do is smash a dispensary on bogus charges and then have the standing case law subject to even more
judicial scrutiny, etc...
The only functional thing this does for Missouri residents is, assuming the state sticks to their word, someone could get the kopsch called on them for say, an unregistered MG being fired, cops show up, and see that nothing bad is happening, and instead of going "WHERE ARE ZE PAPERS?" and then ratting on the guy to the feds when no papers appear, they just say "be safe, have a nice day!" we hope.
-Mike