Which they would determine using... an automated algorithm which decides which category a video should be in, and comparing it to the category chosen. At which point...
Well, it's both a challenging production task (which Google is great at) and a learning from streaming data task, which Google also has some experience with e.g. news. The latter is certainly a interesting challenge, but many researchers are already working on it.
They have more data than they do now for machine learning, and a better PR story.
i.e. uploaders can't be mad about the categories, because the categories are chosen by the uploader.
Uploaders can be mad about Youtube double-checking the categories and getting it wrong... which is less likely to happen if they have better data for machine learning.
Ultimately the determination is still made by the machine learning process, so you're describing extra work to provide an interface of dubious value that will be used more to misrepresent video content than to provide useful signals, and it seems that customer support related to this would increase dramatically.
I think Google relies overmuch on questionable ML in most areas, but in this case, the alternatives are either ridiculously expensive, or easily exploitable.
