Maybe they just know that the movie really, really sucks and they don't want the news to get out.
A classical music station used to broadcast the entire original radio show "War of the Worlds" narrated by Orson Wells once a year, on Halloween. It sounded very real.
Of course they didn't have me fooled for a minute. I knew that, if creatures came from outer space, they wouldn't land in New Jersey, they'd land in Los Angeles.
Typically that is the reason. Or Spielberg may be trying to prevent spoilers. E.T.'s image was tightly controlled using non-disclousre agreements, so it would not be the first time for S.S.
IMDB reviews are running 7-1 very positive, but of course those could merely be Dreamworks staffers.