It may not be that they kept their mouths shut - it may be that they did not say anything so unacceptable.
We also once understood that these peoples' competence was entertainment and we did not consider them to be deep thinkers, paragons of virtue and, for that matter, even "artists."
You said: "The old Hollywood and the older, pre-rock and roll popular culture was decidedly pro American."
Does disagreeing with President Bush make one "decidedly anti-American"?