They may not be CASH tickets, but they indeed are revenue tickets. If it's an Asia Miles ticket, then the outstanding liabilities (miles, which have value) I suspect is decreased when the pax flies and the miles are burned. If it's for AA or BA or whoever, CX must receive compensation (not sure how they do it, maybe they net it against MPC's flying on AA or BA, but either way it's not free).
From an accounting perspective, I'm not sure how it works - might be different also between IFRS and US GAAP. But either way I think the general idea would be to book the award ticket in as revenues at some rate (similar to what hotels do with comped rooms)...and if it's AA, then presumably there would be some cash transfer from AA to CX (or some other compensation), whereas if it's Asia Miles some liability somewhere for CX decreases.
Award tix are not free just because cash doesn't change hands!