I don't see any issues just showing the video. It's on Youtube, so everyone can see it anyway. If people don't want their videos (or ideas/knowledge)being used, they shouldn't share it.
Naturally all copyrights go to the owner.
But to be very honest, most people don't even ask these things and just simply do. I have seen many presetations (commercial, educational en public) were people show these things all the time?
Nobody really cares about it as long as you don't use it to directly gain profit (revenue or whatever) from it.
(so it's only used for educational or illistrational purposes)
As far as I know you need to ask Youtube, and not the youtube uploader. If you're really in doubt and hesitating, just ask both Youtube and the uploader.