No, actually it isnt... Some classes only allow DOT legal street tires (drag radials) and there are some classes for slicks only...

Some cars dont make enough power to spin slicks, so they use drag radials, some cars make just enough power, to use either or and not really make a big difference... My car for example, has plenty of power to run on slicks, but also lacks just enough, that I can still cut some pretty decent times on drag radials, without breaking as much shit... Then you have some cars, that just make gobs of power, that cant efficiently put it to the ground on a radial tire... that is where the slick comes into play, and from there, its all about sizing...