Because the tendency is always for the powerful to protect each other, especially against the lumpenproletariet (that's us).
Perhaps they have a bigger, more insidious plan?
http://www.cfr.org/pub7914/press_release/trinational_call_for_a_north_american_economic_and_security_community_by_2010.php
http://www.cfr.org/pub8104/press_release/task_force_urges_measures_to_strengthen_north_american_competitiveness_expand_trade_ensure_border_security.php
It's all coming together.