This gif of the end of the Floyd Mayweather, Jr. vs. Victor Ortiz bout on HBO pay-per-view appears to back up the case of those claiming that referee Joe Cortez had not yet restarted the bout at the point where Floyd Mayweather landed a two punch combination, knocking Ortiz out. But there's more to the story...
The fight had been stopped after Ortiz threw a headbutt to the chin of Mayweather, Cortez took a point and then the knockout came. I originally thought that the fight hadn't been restarted but video of the ending does show Cortez restart the fighters. He says "let's go" and motions them together.
For whatever reason, Ortiz then turns to ringside officials and is not looking when the punches land.
From the video evidence it does appear that this was a legitimate knockout. One can question how "sporting" it was of Mayweather, but it does appear to be legal.