No its not illegal. Anyone is allowed to install it on their car.
You only get a ticket if you are driving with it, its a distraction to other drivers on the road, a safety thing.
If you are caught, u have 1 week to remove it, if caught again its a fine and the cop can physically use his hands to take it off ur car and any damage done to the car his not at fault.
Trust me, I know because I installed a few on people's cars while I was working. You are still allowed to have it on while parked at car meets and shows.