For me, the telling line is:

...Jalbert had failed to alert authorities before crossing the border, because he had a gun and because he had a criminal record.

It may be an undefended border, but it's still a border. September 11 has changed a lot of things.

If you want to go gopher hunting in Texas, you'd better get the paperwork filed with the right people.

...there was a long-standing custom in Pohenegamook of Canadians darting across to the U.S. pumps without first clearing entry at a border crossing a kilometre away.

That custom existed due to the good graces of the US, not because of any inherent right to "dart across the border" at will. Again, 9/11 has changed things. This ain't your Pappy's border anymore.