I could be wrong, but I think it is to control the spread of thistles, which are considered a noxious weed in many places. I remember reading about a homesteader who was faced with an order to remove acres of thistle overgrowth from her land... at a horrendous cost.

thistle is its own curse word in certian places. i've seen several yards that just stopped getting mowed because the thistle just kept growing in stronger. ever tried to throw a bale that was about half full of thistles? not fun stuff....