Ron Paul generally votes against Free Trade treaties because they mean giving up sovereignty. He certainly thinks that it's alright when the supreme court takes down a law for being unconstitutional but that's not what happens in Investor-state dispute settlement.

This is not about suing government agencies for violating the law. It's about suing states for having laws that go against things written down in a free trade treaty in secret international courts without juries. The Canadian government thought that a pesticide from Dow was harmful so they forbid it. Then Investor-state dispute settlement allowed Dow to sue over the lost sales.

Ron Paul also favors states rights which don't exist when some international court can simply punish a state for it's policy decisions that are in agreement with the state constitution and law.