My '03 Explorer had the same plastic rivets. I took a nail punch and hit the center part of the rivet until it popped out through the back of the rivet. Then, the remaining part of the rivet pulled right out.
It's hard to explain, but give it a few whacks with a nail punch or other suitable object and you'll see what I mean.
I don't know anything about WA's laws regarding front plates, but I can tell you that it's a non-moving violation in most states. Thus, the police can only pull you over if you're already doing something else (such as speeding).
Back when I lived in IL, I took my front plate and front plate mount and beat it with a hammer. Then, I tossed it under my seat. Anytime I got hassled by the cops I'd pull it out and tell them that someone backed into my front bumper the other day, and damaged my plate and mount. I'd explain that I had a new plate on order from the state, and front plate mount from the dealer. They never bothered to verify.
It may be childish, but it works ;-)