Would you rather he arrested William Taft, Thomas Dewey, Richard Nixon or prominent Republicans? How about arresting you?
Grant the president the right to jail 'traitors' without trial and you're leaving yourself at the mercy of his definition of just who a traitor is.
I completely agree. But that is effectively what granting the President the authority to suspend habeas corpus does. That's why I can't agree that there is any ambiguity as to who has the power. The founders would not have allowed the executive to have that power.
The SCOTUS and the courts are not the ultimate guarantor of our liberties: we are, through the most effective check on the abuse of power-- elections. If the arrests had gone through, Truman would have been held accountable in '52, or even earlier through the impeachment system. If the threat had been deemed credible, then Truman would have likely suffered no penalty, just as Wilson did not during WWI.