"The problem comes in when the "because (Z)" clause is "because it hurts me/makes me unhappy/makes me lose trust in you/etc" because that is a moral judgement. Yet many interactions by parties in relationships have such messages."
Absolutely right. For sure.
The verbal self-defense system that I teach is intended to provide people with an assortment of strategies and tactics and techniques from which they can choose, with the idea that they will then be able to stop just reacting on the wing to hostile language -- or to nonverbal hostile communication -- and start making deliberate strategic choices. One of the possible choices is the three-part message; it accomplishes many things, and is tremendously useful. But it doesn't fit every situation.
When the only justification you can offer for asking someone to change their behavior is "because it hurts me/makes me unhappy/makes me lose trust in you/etc.," then a three-part message can't be constructed and that technique is unavailable. But there are other choices. You can just Level, and say honestly, "When you [X], I feel distressed, because it hurts me.... etc." That option is always available.
The hazard it poses is the vast array of cruel responses it invites, and the destructive scripts that go with them. Like....
"So it hurts you. Tough. Grow up, why don't you."
"It doesn't hurt you at all, you're just mad because you can't get your own way all the time."
"That's a lie. Nothing about [X] would hurt any sane adult."
"Oh, yeah? Explain that! HOW, exactly, does it hurt you?"
"Oh, for god's sake, you're such a wimp! EVERYthing hurts you!"
"Really. It HURTS you. So?"
Moral judgments -- "When you [X], I feel angry, because no decent human being would ever do that" -- offers the same opportunities. "Oh, yeah? Since when are YOU an authority on decent human beings?" "Really! Define "decent human being." And so on.
All of this focuses the disagreement squarely on the validity of the "because" component, lays the burden of proof for that component on the person making the complaint, and takes all attention away from the item of behavior for which change is being requested. This is very convenient for the person who's being complained to.
Finally .... I'd advise caution, always, with saying that something "makes" you unhappy, "makes" you lose trust, and the like. If that's true, it's much wiser to keep it to yourself; it's unwise to acknowledge to another person that they have the ability to control your emotional reactions.