IANAL but from my experience people get in trouble for sharing (or uploading) the copyrighted material (but not downloading per se). Not sure if this happens due to some law or because of the tracking/software limitations.

Basically, if you use a torrent client to download a movie, it is in your interest to not upload/seed it because that's when you get caught (usually).

Because Popcorn app forces you to seed the movie, it is clearly illegal in the US and will get you in trouble if you can't hide your tracks.

PS: It is legal to share/stream/upload your own content. As long as it doesn't violate some other law.