Perhaps the term used to be common in British English, at about the time all the colonial dialects were forked, but has since passed out of use here only. Unfortunately for that theory, though, the oldest cookbook I've got (a facsimile of the 1805 Hannah Glasse) doesn't mention cups. It just says "half a pint". Or perhaps it's not really Commonwealth-wide, but only occurs in Commonwealth countries "near" (in a trading sense) to the USA, and influenced by them: Canada, Australia, NZ. Are recipes stated in cups in South Africa? India?

Peter