It is nothing more than testing the revenue-model for the future.
It does not surprise me the slightest, I called it backthen in 2010 and it is inevitable process. All driving games - especially driving games - will transform into "iRacing" model during next-gen to some point at least.
You will be buying barebone game and for real content you will have to pay (significantly?) more. All publishers are testing that ATM. Microsoft (through Forza), Sony (through GT5), Codemasters (through Dirt 3 and F1 - see my explanation below), EA (NFS, Burnout), etc.
For example just take current stream of F1 games - basically, we got 3 games in 2 years as a 3 separate titles, but with almost the same recycled content and few new features. Compare 2010 and 2012 and tell me that all we got during that 24 months couldn't be simply updated via online-update or DLC? GT5 is a clear showcase it can be done. However, F1 is the greatest example of same scheme as these "Seasonal Passes", just done as retail-disc. One can argue how "new season equals new game", but deeply inside you just know it is not. We showed 150 euros to Codemasters through past 24 months for 3 games that are pretty much the same game, right? And we got what? Pretty much the same 24 cars on 26 tracks (2 tracks out from 2010 to 2012, 2 tracks in) and few new modes and 3 new drivers. And that is it. 150 euros.
There is no way in Universe I will support this with my wallet, I learned it the hard way with FM4 LCE + Seasonal Pass + Porsche Expansion DLC. And it was my last investment in such scheme. Despite I am really looking forward to Horizon, I also really wish that "Seasonal Pass" bomb hardly, simply because Microsoft has to understand how this aggressive and wrong way of treating the content is deeply flawed. This particular "Horizon Experiment" will be the testing ground for future Forza titles and it just shouldn't be helped to succed.
Not to mention how they will probably earn another zillion with in-game car-unlock tokens (also patented by EA), but that is another story..