Being Right And Being Direct In Saying So Isn't Always The Best Persuader
Directly confronting a wrong belief people have tends to engender defensiveness, not change.

And putting out the message about what's wrong gets that message out in a way that may increase its reach and pull.

Old RPM Daddy sent me a link to a Science 2.0 piece by Hank Campbell that suggests that pro-vaccination PSA may do more harm than good because of the way they put the message across:

In 1948 [Lyndon Johnson] was trailing the incumbent in his first Congressional election with just over a week to go. He instructed his campaign manager to spread the rumor that his opponent's family wanted him to stop having sex with their farm animals. His campaign manager said it was crazy, no one would ever believe it. Johnson replied, of course not, I just want to make him deny it. He closed a 10-point deficit and won.

Making companies publicly defend problems they never had - "have you stopped beating your wife?" - is a time-honored tactic.

So it seems like it would be a bad idea to proactively declare you have stopped beating your wife. Yet that is what vaccination PSAs debunking a link to autism might be doing.