You don’t have any idea if it was uploaded legally or illegally, especially since it’s a movie clip, not the full movie.
Youtube has a mechanism for content owners to report violations if they wish to enforce their rights. It’s not your issue to worry about, since much of the stuff that’s posted is either fair use, public domain, or the rights owner simply doesn’t care about enforcing their rights in this particular situation for various reasons.
The rights situation in a particular case is usually so hard for the end user (you) to figure out that the end user cannot be expected to figure it out in every case where they might watch a video.
If this type of thing is going to bother you, then I would suggest you stop watching Youtube and seek help for possible scrupulosity.