khert87The short version is that there is no such absolute rule. -Ing modifiers preceded by a comma are most commonly adverbial, but we have to use the overall structure and meaning to determine which part of the sentence they are modifying.
In this case, it may help to strip out all the modifiers and look at the sentence core (always a good idea, really):
Attorneys have occasionally argued that misconduct stemmed (from a reaction*), but the perpetrators are told that they are not responsible.Clearly we lose a lot of meaning here, but we now better see how the sentence is built. The conjunction "but" links two otherwise independent clauses: "Attorneys have argued . . . " and "The perpetrators are told . . . " Since the modifier in question is after the linking word "but," it has to apply to the second clause. Technically, it isn't modifying "perpetrators" but rather the verb "are told." However, when we modify a verb, we have to be talking about whoever is doing the action, so it comes to the same thing. For that reason, when we have a modifier that precedes the clause it modifies, we can usually get away with treating it as a noun modifier applying to the subject, even if that's not strictly accurate.
*(I have "from a reaction" in parentheses because it is technically an adverbial modifier for "stemmed," but that verb is hard to read by itself: what would "misconduct stemmed" mean without the modifier?)