Why are we being penalized suddenly for paying with a credit card?

I have been on autopay (using my credit card) for ages, which gives a $5 discount. I was just informed today -- and only because I called T-Mobile, I might add; I never got a notice about this -- that in order to qualify for the discount moving forward I have to use a debit card or connect my bank account.

It was bizarre, because the rep on the phone kept assuring me over and over that my information was EXTREMELY safe with T-Mobile. I told her I don't care, I want to use my credit card like I always have. It just irritates me that I'm suddenly being penalized for this out of the blue.