It's becoming a negative label because they aren't as good.
I'm not saying it will happen, but it's possible to imagine a future in which AI videos are generally better, and if that happens, almost by definition, people will favor them (otherwise they aren't "better").
I'm not on Facebook, but, from what I can tell, this has arguably already happened for still images on it. (If defining "better" as "more appealing to/likely to be re-shared by frequent users of Facebook.")
I mean, I can imagine any future, but the problem with “created by AI” is that because it’s relatively inexpensive, it seems like it will necessarily become noise rather than signal, if a person could pop out a high quality video in a day, in which case signal will revert to the celebrity that is marketing it rather than the video itself.
I'm not saying it will happen, but it's possible to imagine a future in which AI videos are generally better, and if that happens, almost by definition, people will favor them (otherwise they aren't "better").