How is using the legal system "abusing" the legal system? He makes his arguments for his position, and the court rules, one hopes, according to the law.

The law should be a stable foundation on which society can rest. If the treaty in question does actually govern property rights, and the people who bought property relied on the fact that it does so, and has done so in previous cases (thus establishing precedent), then it is unfair and abusive to change the status quo out from under them arbitrarily by popular opinion. That's not justice, that's tyranny by the majority.