Talk L8 has ceased, but if you choose the "Freechat" option on Telstra plans, you can still have 15min calls for free to other Telstra mobiles after 9PM (including to prepaid mobiles).
Telstra was having MAJOR problems with call congestion after 9PM, especially in Western Australia. Although I think the practice stinks, all the carriers cater for contract customers first. Prepaid customers have a very low return for the carriers, so they piss them off first. :shoot2:
Why? Well, for example if I just signed up for a Telstra contract, and tried to make a paying call only to find the airspace congested, then I could technically go to Telstra and demand my contract be cancelled without penalty, as they did not provide "Every reasonable measure to ensure available capacity". And quite possibly keep the handset if I pushed the matter far enough. The carriers don't want this, so they push the lowest revenue customers first.
Hope this clears things up for you :)