Just playing devil's advocate, why would someone pay the standard monthly price if they could get the annual price discount and cancel after a month?
Presumably the current arrangement is some kind of creative accounting exercise though, and such a pro-customer policy might blow it up.
What you just have done is eliminate the monthly price. There can never be an annual contract anymore. That seems beyond silly. Is it really Adobe's fault for you trying to either be sneaky or simply malicious?