Here is several things coming to my mind:
First of all I dont think videos are stored in an actual blockchain. The file size would make that very impractical. As far as I know the blockchain only stores some information of who posts what. So the post stays in the blockchain while the video could get deleted. Also sites like d.tube and steepshot are basicly indipendent sites so they can hide stuff when they want to but the post would still be trackable on steemit.
Now, let me just say I am no expert on this topic by any means so I might be completely wrong with my conclusions. If somebody is better informed I would also like to hear it!