Seems like any place I can be (I live in WA, too) is a place of public resort, accommodations, assemblage, or amusement. If the public is allowed into a dentist's office (even though it's not an open house, they do treat the public), I think it has to abide by the laws that apply to places of public resort, accommodations, assemblage, or amusement.
And, OP, there was a time when I was willing to follow the rules like you did, but now there's no way I would not go to the waiting room & nurse my screaming babe. What are they going to do about it? I have rights, and my baby has rights, and I refuse to leave them at the door when I (a paying customer) go to the dentist (or any medical professional).