The court will always attempt to find a way to legalize federal activity by using the most narrow ground, unless there is some narrower ground making the federal activity unambiguously illegal.

One rule of jurisprudence is to decide cases on the most narrow ground to leave room for future decisions. The court has historically understood that if you do otherwise, you'll lock law within the zeitgeist of an era and either have to die by it (the country will fall apart because the precedent doesn't fit the times) or they'll have to regularly violate state decisis, which would make the court look more political than it already does, which also injures a country by effecting how the people view the courts and law, also leading to collapse or empire.