The first question you have to ask about Telstra's decision to throttle mobile data instead of charging exorbitant rates for excess usage is why it's taken so long? The second question you ask is why no other major carrier has beat Telstra to the punch on this?
Telstra last night announced that it is building the backend to allow them to shape mobile data usage when a customer exceeds their monthly allowance, and it should be available by the end of the year. Should a user wish, they'll be able to top up their data to resume full-speed browsing.
In addition, Telstra is working on upgrading its SMS notification system so customers can be warned of their mobile data usage in close to real time.
The ACMA has already come out in support of Telstra's announcement, as will pretty much any customer who's received a monthly bill pages longer than they expected it to be.
But the real question is why it's taken any of the major telcos to take this long to do this? Throttling is a standard practice in the fixed broadband space, so it's not a surprise that it's an approach that both works and is welcomed by customers. Hopefully Telstra taking the first step will drag Optus and Vodafone along from the excess usage fee dark ages.