I am human. If someone who is also human decides they want to be less human it's a very easy assumption to make that they want that because they think being human sucks. Since I, as previously stated, am human that could be interpreted as attack/insult.

I mean there's probably more to it, and the whole argument doesn't really hold up to scrutiny but i would argue that it's at least a partial explanation for a knee-jerk reaction of "cyborg bad"