Do you think Australian ISPs haven't tried to offer US-style
flat-rate services? Of course they have. And they get destroyed
in the marketplace.
Here's the thing that metering gives you: it stratifies the
marketplace. It gives you two classes of customer.
One class is customers who know they can live painlessly within
the boundaries of whatever quotas you're offering. They don't
complain, they just pay their flat monthly bill every month
and get on with their lives.
The other class is customers who do so much P2P that the
imposition of quotas is a painful and unwelcome experience.
They whinge and bitch loudly about how awful their ISP is,
and migrate en-masse towards whichever ISPs are providing
"unlimited" services. The only people who truly care about
"unlimited" are the ones who know they can't live within any
limits.
That means "unlimited" ISPs almost exclusively attract the
most voracious, least profitable, noisiest, most difficult
to support, loudest complaining customers. And the metered
ISPs cater for normal folks who aren't like that.