This is a bug in the launcher, not an intentional design to stop you from playing offline (I can launch PA2 offline ok).
A few people saw similar issues with BTTF.
These threads might have more information that will help.
There seems to be a problem with the launcher where if you're online but it can't contact the Telltale domain, it will just sit there "loading" forever. If you're really offline and there's no connection available for the launcher to attempt to reach the Telltale domain, it will load in offline mode.
Are you definitely disconnected when you run the launcher? Try turning off your router or otherwise completely disconnecting from the internet, then launch the game. Note that you do have to activate the game online once before you'll be able to play offline.