I believe TMZ.
From the article:
"We're told Will was aware there was talk about asking him to leave the theater. During one of the commercial breaks, we're told Oscars producer Will Packer walked up to Will and said, "We do not want you to leave" ... this according to our sources.
Will Packer walked up to Will Smith just after 8 PM, around 35 minutes after the slap. That's when he told Will he could stay, according to our sources. Will won the Best Actor Award around 5 minutes later."
Will Packer is a friend of the Pinkett Smiths. He worked with Jada on some projects. I also stand by my theory that he KNEW Will was going to win Best Actor beforehand. He wanted his
friend to get his chance in the sun.

Will Packer wimped out from doing his duty to the other attendees. He wasn't impartial. He let Will stay, IMO.