Mine's done the exact same thing, I don't believe it's a case of 'steering too hard' heck, I never had a cheap $20 AM radio do this, the stoppers are moulded into the case and IMO a few probably got brittle during the manufacture process, I tried a warranty repair and the Australian distributor grasped onto the 'physical damage' excuse with amazing speed (even though the diagnosis took 10 minutes, they had my radio for a week) The fact that my radio has not a scratch on it and that it's kept in a full padded case is obviously irrelevant.
The radio's are still usable, just have to be aware that if you over push the wheel it will click and over extend. It's a great radio but this is a fault that is happening more and more, and until Futaba acknowledges it could well be an inherant design fault, you're at the mercy of their warranty department, and they may well do what the Australian distributor did to me, hide behind 'physical damage' and take the easy way out.