It has been considered "Legal" because it's never been contested but it's never been lawful, it's still on the books! no amendment has ever been made, anything can be made "legal" if some guy writes it on paper and it's signed by some other authority figure but that doesnt make it lawful.
It has been contested. Constitutional law if formed when something is contested on constitutional grounds, and it makes its way through the court system, usually up to the level of the Supreme Court. Once the Supreme Court has decided, then then that's it, unless there's a constitutional amendment, or the Supreme Court can be persuaded to revisit it. The constitutional legality of paper money was settled in a series of Supreme Court cases:
The most significant being the last,
You may agree with Justice Field, but for all intents and purposes, a 8:1 supreme court decision, and subsequent usage, means the matter is settled short of a constitutional amendment.