OMG, they did it!! I've been saying this about track DLC and that is to fully implement them into single player career mode and they did it!! So happy that this is now possible!
I was thinking about this, and I wouldn't be surprised if releasing tracks for free was their plan from the beginning.
I mean, in previous games when they released a new track they had to make a split online hopper just for it, and those were typically as good as dead within a week of the track's release, because predictably enough not many people like racing on the same track over and over again. Putting the tracks out there for free means keeping hoppers consolidated and online racers playing. That and the expansion to the single player career will keep people interested longer, meaning less used copies on shelves. A car pack alone doesn't usually add to a games lifespan the way this kind of track DLC does, so on those they make their money upfront.
I was happy that we finally got another track, and happier still that it was free DLC. However, I'm afraid that giving us this one for free is for good graces and that subsequent DLC tracks will cost money. Of course, I'd be more than happy to be wrong about that suspicion, but I'm going with "expect the worst and hope for the best."