So considering "if you can't get online you'll be able to get started offline like everybody else with single player mode as you start your DriveClub tour." - why can't PS+ users do the same? Hoping for some added sales from impatient people before the 'free' version goes up, perhaps?
Some of that, and maybe they feel the game is just not presented right without a connected experience (and may end up losing them sales). As basically the "Club" part of the game is not active if they can't connect to the servers, and that's supposed to be an important part of the game.