I believe blatantly copying the work of others is wrong, no matter who does it. The work may be parts, intellectual property, even homework.
It's just easier to get pissed off about it when it involves someone that you know and respect.
Some use the argument that there are only so many ways to make a manifold/shifter/strut brace. I can agree with this to some degree, but when the part looks exactly the same as the original aside from a few cosmetic tweaks, where do you draw the line?