You must know nothing about the trucking industry. I have watched the junk trucks bought at auction in the states and then seen them transported to the border.
If you would stand at the border crossing and look at all the old junk coming from mexico, that are former US companies worn out trucks, then travel to the Ambassador bridge at Detroit and see the difference your would have some comprehension of the subject as it applies to safety alone.
What you're seeing is the difference between long haul and short haul trucks. Apply the same rules to Mexico as Canada and you will see the same trucks.
Read post 26.