We are talking about how to incentivize people here... why suddenly decide that there's magically "no incentive" for something?
Youtube is free to also de-incentivize people for using the wrong category.
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.
What, exactly, is the down side of that?
I think Google relies overmuch on questionable ML in most areas, but in this case, the alternatives are either ridiculously expensive, or easily exploitable.