I just take "the powers that be" to mean "whoever is in charge of whatever I'm currently talking about" so those it doesn't apply to, are automatically excluded.
Eg: " TPTB are idiots for bringing Martha back" : "the people who thought of it/greenlighted it are idiots, those not involved in the decision are not." And same for any other usage.
What I mean is, rather than it encompassing everyone, it's just a quick way of referring to whoever is responsible for the matter at hand, whether it's a writing thing, a production thing, a network level thing... without having to go to the bother of finding out and naming names.
If I said "I hate how TPTB have remixed the theme song", I obviously wouldn't be encompassing the writers in that because it's nothing to do with them. And likewise if I said "I hate TPTB have made Marilyn wear boring clothes", I obviously wouldn't be encompassing network executives in that because it's hardly something they would care about.
I'd like to think people have the common sense to realise that rather than assume they're being ignorant and encompassing everyone/anyone.