What puzzles me on this is if, say the HW circuit was being pumped by its
own pump and CH was off, why there would be no circulation on the CH
circuit
without the a zone valve?

There won't be a substantial pressure difference over the two ends of the
unpumped circuit, so flow would be non-existent or very slight, easily
stiffled by a flap type non-return valve (or the 2 port zone valve, of
course!).

Christian.