“What exactly is the problem with doing things Hashem didn’t asser?”
See the Ramban at the beginning of parshas kedoshim for starters.
Regarding Rav Moshes psak of kibui on yom tov. No, it is not a secret, nevertheless his own talmidim have told me (and in fact was part of the psak I was given) that it was in the category of halacha vain morin kein publicly to the masses precisely because he was concerned with zilzul. someone would say “litzorech the food” cavalierly and say “rav moshe permitted it” when in fact he did not.
Most importantly, it really doesnt matter what we do or do not think of this switch. It is what my posek says that matters to me and what your posek says, that matters to you.