People "roll coal" it because it is kinda amusing to do it, and it is a middle finger towards people they perceive to be preachy.

I accidentally "rolled coal" in my 90s Landrover because I was in totally the wrong gear going up a steep hill. It was amusing in the way of "oh shit! I kinda just blew a load of black smoke in the driver face behind me".

Obviously, I don't do this deliberately.