A few things: first of all, it's not US CBP, but US TSA. Secondly, the requirement isn't imposed on the preclearance facility but on the screening facility, which is operated by Canada. Third, even if the requirement was for the preclearance facility, the rules under which it operates requires that Canadian law be followed. And fourth, even if you don't have a "right" to do something, you can't be denied that thing because of discrimination (nobody has any "right" to enter a particular store, but that doesn't allow the store owner to deny entry to a particular race).