Why? Yes, typically U.S. taxes would be deducted from paychecks. But the goal would be to claw all of that back at tax time, using the tax treaty credit, whereby Canada would actually obtain the tax revenue. As chs has discussed previously, U.S. tax deductions from pay checks together with quarterly tax payments to CRA would mean double taxation during the year and a need to claw back the U.S. tax payments at tax time. So, an arrangement to not have U.S. tax withheld during the year makes sense if an employer is open to that.
I'm under the impression that you believe that the U.S. should actually get the bulk of the tax revenue and that most of the tax treaty credit would be applied to Canadian taxes. I don't believe that's the case given Canadian residency. After all, health care, schools, infrastructure, etc. is not free.