I did some research online....It does not appear that any State has made them illegal. I agree though with what the consequences could be by having the spinner smack your hands all around as you drive.
Thanks to all of you for your input.
Laws come and go over time, depending on the do-gooders feelings about your perceived infractions of their high and mighty standards...like cruising, or gathering at the drive-in, or safe, custom modifications to your vehicle. We must all be James Dean delinquents.

A similar code to the current "1977 revision" shown here was pointed out to us younguns by our favorite officer friendlies in the 60's (yes they were very friendly back then!): RCW 46.37.375(1)
http://app.leg.wa.gov/RCW/default.aspx?cite=46.37.375
RCW 46.37.375
Steering and suspension systems.
(1) Construction of steering control system. The steering control system shall be constructed and maintained so that no components or attachments, including horn activating mechanism and trim hardware, can catch the driver's clothing or jewelry during normal driving maneuvers.
As long as the knobs were in the "stored" position while driving we did not get a lecture. Most of the knobs could be flipped and locked to the inside position of the steering wheel where it would not snag on clothing or tenderize body parts. However, it did not mitigate the terror to the fingers when you let the steering wheel pass through your hands coming out of a turn. Soooo...If you want to try one, go ahead. I would anticipate they are less of an issue causing bruising, breaks and running off the road due to snagging your sleeve with thee smaller diameter steering wheels and power steering in today's vehicles.
I still catch a glimpse of knobs, mostly on sweet 40's & 50's era classics and rat rods around town as well as some sharp big rigs.
I do remember that certain modifications including the knobs were permitted for individuals with disabilities. Doctor's orders were required and the Washington State Patrol had to inspect and approve major modifications. A couple friends had their cars modified with all kinds of levers and knobs; one from a bout with polio, and the other due to "issues acquired" in Viet Nam. They were amazingly good drivers.