Demo racers having user accounts is not viable, for the simple reasons that if they did, they could create more accounts to avoid bans for wrecking or other misbehaviour, and that good racers would be having one of the priviledges of licenced players. S1 and S2 charge for content, and giving Demo users more of that content undermines the system.
That's without going into the server and administrative costs mentioned that might be incurred, for less gain - because more people would be happy with just their Demo accounts.
Sam