Managing an account: Checking or savings account

USAA Federal Savings Bank has implemented a policy where they are withdrawing the funds for online bill pay transaction a day to a week before the payment is to be made. This is an interest bearing checking account. For direct transfer of funds, the money is withdrawn a day before the transfer is made. For transactions where a check is sent to the merchant, the money can be withdrawn from my account up to a week before the merchant is receive the check. Which means USAA has removed funds from being eligible for interest before the merchant actually cashes the check. I contacted USAA to understand how they are able to do this - take my money from me, take the interest away from me before the actual payment was made and all I was told was " that's how bill pay works ''.