Indeed, you say "can I get a Coke please", not "it must be a diet Coke" - or if you do, you probably want to make it clear that you'd not consider alternatives. Similarly, enough companies use actual-must in job offerings (for all or for some requirements), and enough companies are clear about (some) requirements being a preference that expecting candidates to divine which employers do and which don't mean it when they say "must" means you'll loose out on some candidates, for no good reason.