For Washington State... you can own (with proper papers) but you can't shoot it.
As to why? Just how WA law dictates... hopefully for now. There is a bill in the state capitol that may fix this...
You might want to check the new WA state firearm forum...
http://www.waguns.org/
Starts up Monday, March 21, 2011. It has a section that covers suppressors, laws, history and basic information, etc