Penalty for celebration

In the Eagles-Cowboys game of 12 December, DeSean Jackson was flagged for excessive celebration after a touchdown. Looking at the replays, it seems the foul occured before the touchdown. Why was in not penalized from the spot of the foul rather than the succeeding spot? If the NFL really wants to cut down on taunting, they should have penalized this from the 1 instead of on the ensuing kickoff. I recall one Packer game from the late 1980s or early 1990s where there was a flag for excessive celebration after a touchdown that was called back due to a holding penalty. Both penalties were assessed before the next play.