> If your a Canadian citizen with %50+ native blood....
> since it would mean all native born Canadians could just apply for US citizenship on the border
These are not equivalent, and so it's clear why not all native born Canadians can apply for US citizenship at the border. In particular Native-Canadian is not the same as native-born Canadian. It's like the difference between Native Americans and native-born Americans (note the difference in capitalization, if that helps).
> since it would mean all native born Canadians could just apply for US citizenship on the border
These are not equivalent, and so it's clear why not all native born Canadians can apply for US citizenship at the border. In particular Native-Canadian is not the same as native-born Canadian. It's like the difference between Native Americans and native-born Americans (note the difference in capitalization, if that helps).