There is not a lot you can do, the matter is entirely at their discretion.
Once you make a late payment the decision to rescind your promotional rate offer has always been a option that is available to them. You are lucky in some respects, that they have at least decided to reinstate the offer. !
Advise that you set up a DD with your card issuer to cover at least the min payment due on the CC, so it always arrives before the payment due date on your CC account, this should prevent you overlooking or making a future late payment...hence avoiding any future charges.