Is AT&T actually evil? Entrapped in a full month bill.

I knew that my promotional discount was ending on the 29th, and my billing period ended on the 24th. I called a couple weeks prior to schedule a cancellation, they said, "no, wait, wait, we might be able to get you another promotion but you have to wait until your current one falls off, call back on the 30th." Which I just did, not only were they unable to get me another promotional discount, but they call said they won't prorate my bill. So now, they want me to pay for 25 days of my full service price, even though, I successfully cancelled today, and they basically just entrapped me by telling me to call back after my discount expires. It's truly evil. So I'm switching to Xfinity.