While users have not the obligation to investigate the source of a movie in detail, depending on your juristdiction there are laws about "obvious illegal sources" (like sites where one can download crappy copies of movies still in cinema for free, extremly shaddy ads, covered by public media as being illegal). For example if Netflix turned out to not have a license in my country I could argue that I as a user could not have known that since they made looked and acted like a regular respectable company. If on the other hand I download a torrent containing a recently released movie for free and an accompanying txt file allowing me to "share it to the world", my chances of defending that in front of a judge are not the greatest.

I'm not sure if your first sentence is very accurate. Where did you get that information?

Well most of the people prosecuted for piracy, maybe even all of them, were because of uploading/copyright infringement, it's usually a very gray area of just downloading/watching a stream, and I've never seen a case of it being illegal per se.