To me the point is to remove the negativity. They don't want to be the targets of the same criticism as FB. It's also seem to be a cultural trend in the US that only positive comments are accepted.

A lot of comments here already point out that YouTube’s own videos are regularly downvoted to oblivion. Also that in this very sensitive political climate across the western world gouvernements and big corporations don’t want to appear rejected by the opinion. I tend to think it was an important argument for this decision