Some students believe that they are entitled to a grade of A no matter what garbage they produce. For these students, then, actually reading instructions might seem like an unnecessary bother.
For certain other students, there may be more a of a matter of principle at stake. These may actually read the instructions--sometimes even carefully--but may believe that flaunting directions with impunity is a great opportunity to rebel.
The truth of the matter, however, is that failure to heed instructions can have disastrous consequences. If instead of following instructions to mark the right multiple choice answers on exam, you deliberately mark wrong ones, that could result in a very low score. If you ignore with impunity instructions that the term paper should be on a topic relevant to the course, you might end up with a score of zero.