I'm not going to rehash what has already been said, particularly the implicit contract law stuffs and why it would not be a particuarly good idea to go after your most loyal customers.
That said, I think somebody over at US probably has a bit of a problem on their hand. That somebody would be the process manager for things like this, since US has not (in the past) been really keen about verification and validation of who is targetted for what. That's their fault, not the DM member.
As has been pointed out, anybody with a little bit of time can find all these things on their website. And the contract law analogy applies from there.
As for the loyal customer: this airline needs revenue a whole lot worse than FT members who can spend thousands of $$ with another "airline of their choice" need to Get Out More.
Edited to add:
As soon as they define "improper use of promo codes" in the T&Cs, they might have a leg to stand on.
That, and the more I think about this, here is what I think happened: somebody at US finally realized that flyertalk exists and/or "found" the bonus code thread.
Since Randy does have a good working relationship with the airlines, you might expect that he'd post something like this. If US really saw widespread "fraud," they would have sent an email directly to their membership list, as opposed to communicating the "threat" thru the House of Miles.
Personally, if I lose one solitary mile, I pull all further bookings from US, and make the standing order in my group against booking US (due to standby changes) a permanent one. Is one cockroach worth $50-$70k/year?
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Saving the world, one clue at a time.
[This message has been edited by ClueByFour (edited 10-15-2002).]