From the document provided in TFA there seems to be a probable explanation (see pg. 55):
Australia — A place that’s either very near or is New Zealand where more
than half of Valve’s employees were born.
That's the funniest part too, they don't sell their hardware in New Zealand either.
Hence my conspiracy theory! My wild assumption is that they don't want to deal with our consumer law, and/or they want to punish us for the ACCC forcing them to respect it wrt. Steam.