It seems to be a Oneworld core tennet. All of their FF plans have this rule (but some don't enforce it). Recently AA has made the rule more prominent, so they may start enforcing it soon. QF and BA seem to be the strictest (presumably it is their rule). The irony is that the SCs expire, so if you lose status with enough SCs but not the 4 QF segments, then it is gone for the whole of next year (as the SC level starts from 0), not enough to fly 4 QF again and resume the "suspended" status. Though this is likely to be comped upon request in the right circumstances.

The case can be made that offshore WPs cost QF less than those based in Australia; they won't be using the MEL F lounge for weekly MEL-SYD flights, for example And QFF does get cash for selling those QFF points to the other OW airlines. Last year QFF was the most profitable product of QF, so you'd think they'd want to encourage it.