I am so sorry to hear this. Please make sure that next time you make your decision right away, especially if you think that you are most likely to accept the offer. It could be illegal, and you could file a lawsuit if they rescinded your offer while you have already committed. I hope this will be a lesson for the rest of us. Last year the same situation happened at the UA: They minimized the amount of stipend to the applicants who didn't accept their offers "on time" although their offers showed that they still had time to make a decision.
Disclaimer: I am not a lawyer, this is just my opinion.