SMS are still working out all the details but it sounds like 'nothing will be locked away forever' - what that timeline turns out to be I don't know. I can tell you those are not core career cars.
Obviously some are pointing the 'evil' finger at them but given that these guys are not experienced publishers I do get where they're coming from. The devs and us WMD always wanted a special commemorative edition, and SMS want to get an indicator of how many copies of the game they might sell (and should therefore produce) - the best way to do that is to preorder, and the best way to sell preorders is with a limited edition.
So those who want the steelcase and book or want to support SMS can go and preorder/buy the LE, those who get the normal version should eventually be able to get all the cars anyway, and if you're already a WMD member, hold back for future announcements regarding how your discount/free copy works out. It's not an easy process to work out and they've got loads happening atm so we just have to be patient. They've made games before, but never self published.