The argument that BitTorrent had no legal applications is invalid. If people kill each other with guns, why would you put the blame on the gun company?
Popcorn apparently, if the MPAA is correct about the source code we now can't read, specifically helps users pirate movies.
To avoid cranking the discussion up to 11, let me recast your analogy to, "if people break into houses with crowbars, why would you blame the crowbar company?"
I think under the MPAA's argument, Popcorn is like a crowbar sold with a GPS device that finds unoccupied houses to break into.
