Not sure they are illegal.
If they were, why do "Tugs" - used for shunting trailers - have them fitted as standard? Same for road registered Fork lift trucks, Agricultural Tractors & so on....
Not to mention they are common additions to trucks at most places I've driven?
Personally, I don't see the need, the manufacturer already fit a steeering wheel!
In the main, they go well with Blue LED lights, Furry Dice and all the other **** that some drivers like to decorate their cabs with.