Yes, for non retirement accounts there is a 15% withholding for tax; however, when you file your taxes, you can claim a foreign tax credit against your US taxes. So other than the loss of use opportunity cost of those withholdings, there is no negitive tax consquences of these withholdings.
Yeah, they used to do it to both types of accounts, but since PGH converted to a corporation, the payouts are no longer classed as 'distributions', but rather as dividends and are no longer subject to the 15% Canadian tax if they're held in a retirement account--Thank God! I don't know how much I've paid out to the Ottawa government down through the years. Thousands!