Edit 2023: This post is outdated. We only apply withholding to US source based income, this has been the case for multiple years now.
This mean that this withholding tax should apply only for sales to U.S.A. customers right?
We've seen this part of Steam's terms. We've consulted with multiple accountants and lawyers about this for our company and they advised that since we are a United States company we have to apply the withholding rate to all income send to you from our bank account.
I don't know what Steam's arrangement is, they may have other entities in other countries to work around this, or they may have interpreted the tax law differently.