they take the money when they send you the bill

No they don't. They send you the bill and say words to the effect of "unless you disagree with the amount, we will take it out of your account in XX days." Seems very reasonable to me. Unless the amounts involved are mega you are not likely to make any significant interest in the few days between when the DD is taken and the 'last permissible day'. Sorry, but I just don't understand that reasoning.