The only footage I have of the incident is this GIF, which starts with Cortez motioning something to the two fighters. (I've been alerted on Twitter that the fight had officially been restarted, per the post-fight press conference) Ortiz steps toward Mayweather to hug and apologize for the butt. (It should be noted that he already did so while Mayweather stood stunned in the corner.) Floyd brings his hands up as if to defend, Ortiz embraces and steps away, and Floyd clocks him with a vicious left hook. Ortiz, stunned, stares at Cortez, eats a straight right, and hits the canvas. Fight over.
Now, whatever argument was left about the legality of Floyd's attack is out the window with the revelation that Cortez had restarted the fight. The last thing a referee tells a fighter is to "protect yourself at all times." Ortiz failed to do that, and he paid mightily for it.
My Twitter feed, even after the revelation at the press conference, is still full of those condemning Mayweather's left hook-right straight. Some are calling it unethical; others, cowardly. Mayweather did nothing wrong here. He broke no rules, unwritten or otherwise.
The blame rests squarely on Victor Ortiz. Referee Joe Cortez ("I'm fair but I'm firm!") did an underwhelming job of controlling the situation, but the same thing could have very well happened had his eyes been fixed on the two fighters and not at the timekeeper, who he was instructing to restart the clock.
No, Ortiz shares the full blame here. First, for his very illegal and very blatant headbutt in the corner. Second, for not protecting himself at all times -- not only as he disengaged from Floyd, but as he stared at Cortez after the left hook, hands at his sides, a sitting (standing) duck.
I've played sports since I was a little kid. One of the things drilled into my head at a young age is that you play until the whistle. You play until the referee tells you to stop. I've had a long-running hatred of glove taps and hugging in the cage, and it's for this very reason. You're not playing the game anymore.