Why not?, my understanding of UK law is that there is no obligation for company to sell it's products to anyone. If Scavier et al decide that bad competitive behaviour on the demo servers precludes that person from owning an license that is their privilege.
Having said that, I have to admit I had forgotten about the extra content that can be accessed off line with a licence.
Maybe the solution is simply to ban the users in the master server, (rather than on a server by server basis), and use a more complex log in registration system as I mentioned.ie make no distinction between demo and license racers, if banned as a demo user the ban is still in place even if you should then go on to purchase a license.
I'm a strong believer in previous behaviour being an accurate predicter of future behaviour. I strongly believe that if shown to be a cheat/wrecker in demo servers you should be precluded from being able to use licensed servers.
Also, it's important to remember the promise of better quality racing as a licensed user that is currently being made by the developers. They need to be seen to be taking steps to make good on that promise in my opinion.