Why are we being penailzed for paying with a credit card?

Don't ever give companies like this your bank account or debit card number.  They have too many data breaches and are not capable of securing their networks properly.  Paying by CC is ok since you can stop the card and limit the damage.  To stay with T-Mobile, just cancel your autopay since you won't be getting a discount anyway.  Then have them send you a paper bill and you will have the option of paying by check (most expensive for T-Mobile), paying by your bank's bill pay (next most expensive for T-Mobile), or by logging into your T-mobile account and paying by CC (still expensive for T-Mobile).  In this way, T-Mobile won't be realizing cost reductions on the backs of their customers.