As someone who used to think it was an offensive term, it’s likely ignorance and because it’s often used in a deragatory and dehumanizing way on the internet.

At first I didn’t know what cisgendered or cis meant, but I definitely saw it used to describe a group of people non-cis folks didn’t like very much. Of course I eventually learned, but still had a bit of a distaste due to the initial impression.

Also, I always saw “cishet” as a cheeky way of saying “cis shit” because it was also often used negatively in the places I originally came across the term. Once someone explained it in a comment section I finally understood it wasn’t hateful terminology but instead descriptive.

You can’t stop someone from being negative but at least knowing what the words are meant to mean can help identify a bad person rather than bad word.