I would not be surprised to see the Quiet cars on both Acela and Regional assigned as well, with a price premium, of course.
I'm not sure why Quiet cars would have a seat premium. The only time I see them fill up faster than the other cars is for trains leaving Washington, where the quiet car is the last coach on the train, and thus closest to the gate. Before they switched to the back, the quiet cars were at the front of the train and I used to ride them because they are the last to be filled up.
It seems to me that for every person who wants a quiet car experience, there are 10 people who want to be able to yak on their cellphones. Just my experience.