I will agree that the specified rounding policy is not reasonable for low priced stocks. Your choices are to not short low priced stocks at IB or to use another broker.
This is the way the SBL (Stock Borrow Loan) business is conducted on the street. We find a lender of the shares for you - this is typically an SBL desk at one of the banks, or broker-dealers. The borrow fee we pay them is computed in the same way as we charge you. The stock price is rounded UP to the nearest dollar and then the borrowing rate is applied to that amount.
If you don't like the way they round for interest purposes, take the other side of it. Sign up for IB's stock lending program and get paid double what you expected.
Can anyone (outside of IB) actually verify this? I know for a fact it is NOT true outside of the US. Even if it is the case, why wouldn't IB show customers the effective borrow rate they are being charged?