I put "loophole" in quotes for a reason.

The person in question was arrested and charged with tax evasion and filing a false sales tax return. What he is alleged to have done is illegal.

The state was under no obligation to alert him to the fact that they believed he had committed tax evasion and filing a false sales tax return before they arrested him and charged him with a crime.

What he seems to have expected was an opportunity to pay to settle a criminal charge before it landed him in cuffs. This I should be able to pay to make it go away attitude seems especially prevalent in American society today.

This Montana car registration scheme is, I believe, often referred to as "loophole" not because it's legal but because so many people have been able to get away with it without consequence that it is seen by many as relatively low-risk.

No, it is not correct that using a loophole is always legal. Law systems are not simple and absolute, and intent matters.

Except it was illegal, so it wasn't a loophole. It was something technically doable, not legally.

You can get married in multiple states - though some states may share, most don't share marriage registration. Nothing will stop you. But you're still breaking the law of bigamy.