"Any interest earned is easily off-set by the administrative time it would take to calculate the interest earned on each account."

And where would this be an issue? The Administrators are the ones withholding the funds, not the Merchant, and are responsible for any overhead caused by the policy.

Regardless if the interest is $0.01 or $100.00, the Merchants are losing out on the ability to earn interest on money that is lawfully theirs and can expect to be compensated.

Besides, modern computers can make trillions of calculations per second, so you are overstating the complexity to, seemingly, divert attention from the issue.