Airlines would probably argue that they *aren't* rebates on purchases (or a promise of a future rebate). That implies that the miles have value, which they do not.

Miles are vapor. Which, while it has drawbacks like not being able to freely trade them on an independent exchange, is probably the best situation for fliers. Too many unintended tax/accounting/regulatory/infosec/etc. issues if it were any other way. Not to mention the fact that programs would quickly have to simplify down to a not-very-rewarding rebate scheme. Long-haul F awards at a reasonable mileage level would be a thing of the past.