Bidkat, the answer to your question is complicated.

Contractually, the fine print entitles them to do this. However, there may be provisions of consumer protection law that override this result. Also, that interpretation of the contract assumes that those provisions are part of the contract. A court might ignore them if they are unconscionable (vastly unfair) of if a pax did not agree to them--not everything in the fine print is legit or is binding on you. Finally, even if they are entitled under contract and consumer protection laws, the act of changing the terms of the miles might amount to a bait and switch or other deceptive trade practice.

One caveat is that federal law preempts state law, and there may be specific legislation authorizing this as there is with the unreasonably low baggage loss limitations.

Here is an analogy that you might be interested in. Go into a store like Best Buy or Radio Shak that asks for personal information when you make a purchase. Tell them you'll give them that info for $99.95. Have them agree to this on tape. Then walk out of the store with your purchase less than $99.95 without otherwise paying and tell them to keep the change. Arguably you paid for the goods by providing your personal info. If that doesn't hold up, then neither should the fine print in the airlines' contracts.