There are outlandish things that are forbidden. For example, the DMCA means my ripping of my own personal DVD and Blu Ray discs to HDD is not lawful, which is absurd.
i love how people use things like this to reject copyright laws all together. NO ONE is bashing in your front door and tazing you because you made a copy of a DVD you purchased. in fact, how about referencing one single instance where that has happened. no?
Popcorn time I've never used, but the principle of the thing doesn't appeal either, I want to actually have my library at hand at any given time.
PT is just a torrent client. the movies are downloaded to your computer (or mobile device). all the "streaming" mode does is download the blocks in-order, and download up to a certain amount of them before it starts playing.