Somehow I doubt that's the motive, although that may be the (unintended) consequence.

Miles cost an airline less as a contingent (inchoate) liability than it would actually cost them to fly someone to burn the miles. For one thing, the "contingent" liability calculation certainly incorporates the "breakage" -- the miles that will never be redeemed for whatever reason -- while the actual cost of delivering (low though it may be, perhaps $25 or so per ticket) will always be higher than the contingent liability for that (potential) ticket.