They have external dependancies on them (like name servers being used by another domains) so the new registry operator does not purge them until those dependancies have been resolved. If you look at the domain names most of them are old hosting companies.
Personally I prefer the old way where they got deleted and the affected registrar got an official notification that xyz domain has had its name servers removed because abc domain has expired.
This way seem to create unnecessary clutter in the namespace as these unlicensed domains just hang about but apparently this is what happens in other namespaces and it is the new way forward for .au domains.
It would be interesting to see if one day we get two domains depending on each other so they are both unable to be purged yet they are both eligible to be purged by virtue of both being expired at the same time.
- Anthony