Does Starfleet not brief their Captains before sending them on an errand?
When the mission involves ferrying negotiators, clearly and consistently not. Nobody told Picard that Riva would be deaf; that Sarek would be senile; or that Odan would be a slug.
It makes a weird sort of sense. Negotiators are men (women, slugs, whatever) with many secrets. They have to lie and hide a lot in their line of work, up to and including about their own person. Too much knowledge about them would compromise their mission: if the crew knew, they might let slip. The negotiator really needs to be made of teflon.
DS9 Rocks and Shoals quibble: "How the **** do you use a rock to hide behind a PHASER?" Those are *some* rocks.
Remember ST6 and a phaser vaporizing a kettle but not the contents? It's not an issue of the beam removing soft things and leaving the hard ones - it's apparently an issue of the beam only ever being tuned for a certain kind of thing. If it removes rocks, it won't hurt a person, and vice versa.