A steering wheel knob spinner is illegal in most states except in the case of handicap drivers. The laws and regulations vary by state, and even by county, so it is best to check with your local law enforcement office in regards to local laws. Steering wheel knob spinners are often legal for commercial vehicles such as semi trucks or dump trucks. Steering wheel knobs do not allow for proper road feedback and inhibit the ability for quick responses, especially in the event of an accident. In the majority of cases, a person will have to get state approval before installing a steering knob.
For more info, see How to Install a Steering Wheel Spinner Knob