It's much more difficult to send plant products through the mailing system to Australia because 100% of the mail is screened by Customs and Quarantine officials as it arrives. This doesn't mean that every piece of mail is opened, but every piece is x-rayed and run over by detector dogs trained to detect plant materials (among other things).
The colour x-ray units that are used are quite effective in locating plant material and the consequences for anyone being caught can be severe. I doubt whether it would be feasible for USDA officials to screen the massive volume of mail that enters the US in the same manner that it is done down here.
I was in control of implementing the use of these colour x-ray units into mail centres when the foot and nouth outbreak occurred in Europe (before which we had a single B&W unit to cover the entire Melbourne mail centre- now replaced by 6 coloured ones). In the first couple of weeks of screening mail from Europe only, we seized over 30 packages containing various drugs- mainly ecstacy. Nowadays, during a weeks work we would be lucky to find a single consignment. Doesn't mean that the drugs are no longer entering the country, just that they can no longer import it effectively through the mail.