You're misusing the term "Defending Pokemon".
It does not by default mean "all of the opponent's Pokemon".
It means "the Pokemon that this attack is targeting.
Without any indication of multiple or choice of Pokemon, that defaults to the "Opponent's Active Pokemon at the time of the attack".
The reason Fighting Stadium had to be errata'd was because the damage bonus could be used by an attack that did in fact have multiple targets and so that would have indicated that "Defending Pokemon" meant those same multiple targets.
That's not the case with Allergic Shock.