Looking at the model (or what we know of it anyway) it looks like the publishers are trying to ensure that they get a guaranteed x% back on every sale. So rebates which don't touch the profits of the publishers should be ok. I say "should" because I haven't seen the small print. They could have outlawed that too.
On the legal point, I suppose the one possible claim could be a breach of contract. The "you promised me discounts, so I bought membership and now you're not going to give me the discounts. I want my money back!" approach.
However, I reckon if we check the small print of the contract it'll say that the discounts available can be changed by Fictionwise at any time, without prior notice. That's standard practice so the company doesn't sign themselves up to something the they won't be able to honour at some point in the future...like now. If you want to argue with that clause that the ability to change discounts is unfair then you're into the Unfair Contract Terms Act here and good luck with that one...
Generally the rule is caveat emptor or buyer beware before signing [sic]
However, I freely admit that I'm not a corporate or commercial lawyer, you should in no way rely on my advice and I know nothing about US law and things may be totally different over there.
Infact they are still offering discounts where they can (i.e. honouring the 15% Club discount for the life of your membership) but they're having to withdraw discounts for REP books.
I'm taking a "wait and see approach" as I've only got one year left. I'll see what Fictionwise do once the dust has settled and then decide whether to ask nicely whether I'm entitled to some money back as a good-will gesture.