Because that doesn't make the ball dead. He needs to possess it. If the ball was already stopped and then he touched it then it would be dead. The bean bags were to mark the spots where the kicking team illegally touched the ball (not a foul but a place where the receiving team can take possession).
The signal the back judge made though should have made the ball dead. It was an incorrect signal called inadvertent but that's what replay should have done at that point. The receiving team has the option of any of the spots of illegal touching or where the runner was with the ball when the inadvertent signal was given. The inadvertent signal was the mistake by the official and likely a big mechanic downgrade on this play. I think replay may also get a downgrade because they had a chance to correct it and didn't do that. The right thing here would have been NOT to signal and the play resulted in a TD. But the signal mistake is a huge one for the covering official.
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