If Tirerack has changed its shipping policy and doesn't want to ship to Australia anymore, that's a different matter. However I don't think legally any company in Australia can stop someone importing goods from an overseas company that is willing to sell to them. To do so, I believe a new law would need to be passed and an amendment to the free trade agreement would need to be made.
I would love to know what margins the local tyre retailers have on the tyres they sell. Before we have those facts it is just speculation.