TurboSquid pays withholdings to the U.S. IRS for all artists on a monthly basis, and does not hold any money due to the IRS in interest-bearing accounts. Therefore, neither artists nor TurboSquid accrue interest on the withholdings that we send to the IRS. If you receive a refund from the IRS after turning in an ITIN, for example, the previous withholding amounts refunded will, by IRS policy, be only the "principal" amount withheld, and will not include interest.
There are businesses that want to earn interest on other people's money, but TurboSquid isn't one of them. The IRS gets relatively little from us because almost everyone gets a tax ID and doesn't have to pay withholdings.