A WebmasterWorld thread named Google has to pay interest for adsense payments asks the question, shouldn't Google pay interest on the delay of payment for AdSense?
Most of the members in the thread say no, they should not. And I agree. Google probably doesn't get full payment (including charge-backs) until 30 days later anyway. Plus in the agreement, it specifically says that payments will be sent up to 30 days after the month where they have been earned.
This is part of doing business. A business renders services, then invoices for them, then possibly 30 days later gets paid. No interest accrued.