Everyone's reaction to the play was legit until the ump decided to change his mind after Pierzynski started to run to first. Pierzynski put doubt into the umps mind when he started running to first. Even Pierzynski started to the dugout, granted Paul should have tagged him regardless, just like your little league coach tells his catcher's to do, but this play was made when Pierzynski tricked the ump by running to first.
What I found most humorous was when the umps were being interviewed by the press and they said they found visual evidence that the ball had changed directions on the catch. I don't know what tape they were watching but Paul caught that ball and was the reason why he didn't attempt to tag out Pierzynski.
I've been an umpire for over 10 yrs here in SOCAL and believe me it is very easy to tell when a ball hits the ground. The biggest give away is the sound. You can hear it and secondly you watch the reaction of the players. Players react immediately and you know what happened if in fact you couldn't see it or hear it. In this case all three people reacted correctly, until Pierzynski decided to run to first after he had taken a step to the dugout.