FLOOR! What's the ruling?

I recent had a floor situation that I've never seen before. Our floorman is definately good, but I don't understand how this could be the correct call.

This is at a live SnG satalite. I am in mp and it is folded to the button who raises to 800. The SB folds and the BB moves all in for just over 1000. He gets up and leaves the table for about 15-20 seconds. The button calls then asks for the floor to rule the players hand dead who had gotten up. The floor man comes and the player comes back. The floor rules his hand dead because he left the table, so the button takes him out no contest.

Here is why I don't understand this. How is this any different from someone getting blinded out and then being all in on the big blind? They don't just declare his hand dead and eliminate him, they run the cards to see if he stays alive. Here we have the same situation.

How can his hand be dead if there is no action on it?