You assume wrong. If there is such a thing as an "Aussie ES ADS-B unit
Correct me where I'm wrong. But while ADSB operates on the same frequency worldwide, Australia requires an " Extended Squitter" that is not required in the US. Europe may also be using the "Extended Squitter" version also, I'm not fully across that.
Therefore mode s transponders designed for the US cannot be used as " mode s" transponders here.
This is in part Dick's problem, because the US avioinics manufacturers (which apart from Becker and Trig is basically all of them) are developing US UAT versions before the ES versions. I think at the moment the only options for ES versions are Trig & the late model Garmin 330ES (or an early Garmin 330ES with a software update). King have announced a new modes S transponder which will have an ES version, but its not yet in production. My memory is hazy, but I don't believe the new Garmin GTN 750 has an ES transponder available yet, nor do the Garmin EFIS packages.
So, my presumption is that American based jets aren't likely to have ES version mode s transponders and are therefore unable to operate above F290. This maybe OK from the East, but I have not been in a jet that has traversed Australia below F290. So, they either do have ES mode s transponders, or they too operate under an exemption.