I think they are just being intentionally ignorant on the topic due to their dislike of the system overall and I don't think that is fair of them. There is lots of videos even of YouTube via bodycam videos with many police departments making good use of these cameras to aid in solving crimes. I'm sure there are many articles and maybe even research out there which would show this.

I think it's just a way to try and dismiss the cameras without trying to tackle the heart of the problem. When you have to contend with the fact that the cameras have a lot of useful purposes, it makes arguing against them much more challenging. If you can pretend they are not useful, it may be a way to try to stiffle any productive discussion around them.