From what I have observed, I tend to see two reasons when it comes to those who are against answering homework questions.

1.) Those Who think doing the problem would defeat the point of the homework (nothing was learned, just copied from a perlmonks answer).

2.) Those who are insulted because the questioner wants someone else to do all their work for them.

Both points are valid. There is a big difference between helping with homework and DOING someone's homework for them. I also feel that the second point is especially emphasized when someone tries to dupe you by rewording their homework. It shows that the person thinks their time is more valuable then yours. It is very disrespectful.