If customers must buy software up front, they don't receive the benefits of a subscription model and aren't purchasing cloud computing. With large, up-front sunken costs, they don't have flexibility to switch vendors if they are unhappy. Having already paid nearly all fees, they are locked into long-term commitments with vendors who are typically less responsive.
To expose cloud imitators, customers should ask if they can have a ten-year payment period that's cancelable at any time with no penalty. If not, they aren't doing business with a bona fide cloud vendor.