They probably think that dislikes aren't actually a good signal for whether or not a video is relevant to a uer.

Crappy videos could automatically be filtered out of their search and recommendations because people abandon them early. They may have determined that's enough to help user's find what they need.

It also helps the issue of dislike brigading since you won't be able to "dislike" a video at all.

Not sure if that's the right move or not but I don't think it's necessarily a bad one.