LEOs carry the same type of liability that civilians do on this one. If their seargents are just "looking the other way," they should think twice about having that 870 WP in thei car, but if they have it in writing from an authorized ranking officer, they should be pretty well covered.
That is why I use the DA as the CLEO, he is the one that will ultimately judge whatever I may do with the weapon in question.
Is it possible that the manufacturer's info is referring to states in which suppressors are already illegal? (NJ is one, I beleive..)