Reaction: Given how many Soviet spies seem to have randomly wound up inside the Manhattan Project's "Top-Secret fence", without any actual effort by or direction from Moscow - would it have made any real-world difference if Oppenheimer was a Soviet agent? Once Stalin had 4, 5, or more independent copies of the A-bomb blueprints, what was the marginal value of yet another?

Oppenheimer was clearly leftist, and the best way to slur a leftist, certainly in the 50s, and especially if you had agenda (cough Teller cough), was to call them a Commie, no matter where their loyalties lie. See also: House Un-American Activities Committee.