Not true, the rule states you can't make a treaded tire into a slick. However a tire that is manufactured "smooth" is legal ala the Panther Slick tires.

TO get the most from the Panther Slicks, mount 'em up with some stiff foams. Either take a piece of sand paper and scuff 'em up or take 'em out on the street or concrete and scuff 'em up to remove the shine...then you're good.