I have a very strong opinion about staff NPCs that end up TSing PCs. This could be because of my profession.
I mentioned before that an ethical staffer shouldn't be limited in what they do; however, what constitutes an "ethical" staffer varies from person to person. We have tossed phrases like "ethical" and "abuse" without really giving much in the way of definition, but what I think everyone can agree with is that a staffer: (1) needs to have a set of ethics to constrain their inherent power on a game; and (2) a staffer can use their inherent power on a game that can hurt another player.
People that work professions that either require or often create strong emotional or personal ties are ethically constrained from engaging in sexual relations with their clients. It doesn't matter if that professional is kind, gentle, or ethical, as others would use that word: they are in a position which is of such power or connection that the profession binds the professional to avoid any potential abuse. And while it is a stretch to suggest that a staff member on a game is akin to a doctor or a lawyer in their relationship with any player, it is not as much of a stretch to point out that players are often dependent upon staffers for their engagement with a game.
I am of the opinion that a blanket ban, while constraining the otherwise-ethical staffers in how they may engage with players, provides a baseline of behavior -- an ethical rule -- that can inform or assuage players as to what sort of RP staff may press them into. Were I to run a game, I would probably insist on such a ban. While many good stories may be cut short or snipped by such a rule, I would point out that players can RP NPCs that can engage in TS in a PRP.
And, yes, while I recognize that staff are also players, I cannot overlook how staff have powers and duties beyond players.