That's quite extraordinary — I've registered prepaid SIMs with Telstra, Vodafone and Virgin which have all required registration in order to receive calls and be topped up; a process which Google tell's me via http://www.comlaw.gov.au/ComLaw/Legislation/LegislativeInstr... is the norm.

By law they are meant to (I used to sell them) - depends on the guy behind the checkout though.

Did you pay cash? Did you have to register by phoning Optus before using the SIM?