To be fair, it's not a free ride. The other carriers pay a wholesale rate for the access and traffic to Telstra. So Telstra is able to cover it's costs and probably make a profit (they wouldn't admit that) from third party carrier sales of it's network.
I think that having a private company owning the infrastructure and marketing services on it is not such a smart move. there is a lot to be said for separating the infrastructure from the marketing, eg: One company or Government Authority installs infrastructure and sells access to the carriers who then supply the market for mobile services, land lines etc.
Be nice if they sort out the confusion of convoluted phone plans while they are at it. :)
woodbe.