I haven't been following this story all that closely and I had no idea that you could only enter the gas station from Canada. That, to me, makes the situation even more absurd. I'd bet money that the US guards never checked in since there was an informal policy about doing business in the area located between the two border stations. For the US guards to take advantage of that while arresting someone who thought they were protected by the same policy is extremely hypocritical.
What amazes me most is that this station has operated for so many years when clearly the only reason for its existance is to dodge Canadian taxes on fuel. Not that I have anything against dodging taxes if it's legal.