Because that is what a Yeerk would do. They control sentient beings. You say its just a morph, but still, what happens? You become that creature, THEN you control the creature. If the morph is sentient, then it is no different from controlling a sentient being.
It's impractical, but its morally wrong. I'd feel insulted if someone morphed me or morphed my father. Heck, I'd feel insulted if he morphed my long gone grandmother....