Why are ace people upset at shipping ace characters?

So I watched 2 videos about how fandom doesn't respect ace characters. (Both of these videos mentioned Alastor from Hazbin Hotel btw.) And while I understand that there isn't that much representation for aesexual people and that might be a factor, I don't see why ace people would be upset at the shipping. Unless of course the shippers were demanding that the creators of the show stop making the character ace. That would be pretty disrespectful and mean.