Tony--
While I don't agree with you that MM is the worst DW book (we all know which ones I think are the worst and won't bring that up again), it does have an unusual number of problems.
I don't have any problem with Moist understanding how to get the golden (not real gold) golems to obey him. He is a master of reading people (and thinks) and understanding how to use showmanship to get his eyes. Remember when he got the killer dogs to obey him during the postman's test in GP because he knows that they're the same kinds of dogs he grew up with in Uberwald? Then he finds out after the test is over that the dogs, although the same species, were not from Uberwald at all and therefore would never have been trained to respond to the words Moist used. He succeeded because he has an innate ability to get most living things to believe him and follow his commands.
He could infer that the golden(not real gold) golems would obey him because he knew that it took, as Professor Fleed said, the right person, saying the right word at the right time. He knew how Omnian priests dressed, and when he saw the lead golem's eyes following him he knew that he met the condition of the 'right person at the right time'--all he was missing were the right words. There's no reason why Vetinari, a study of all things historical, wouldn't have known this as well. But, presumably, Vetinari didn't know ancient Omnian and probably would never have dared to lower his dignity by wearing a golden suit. Besides, if Vetinari had done all this and failed, he would have been a laughingstock. Better to let Moist do this for him, since he knew that Moist would never use the golems for hostile purposes.
I think the discomfort most of us feel about the way these golden(NRG) golems were treated reflects our own feelings about how humans or humanoids should be treated. After all, if the entire golem army consisted solely of golem horses or golem cows or golem sheep would you have felt the same level of discomfort of having them treated as property? If you do feel this level of comfort, does this now make you feel that you'll never eat a hamburger or mutton or ride a horse or take a horse-drawn carriage ride again?
It's because most of these golems are in the shape of human beings that we feel uncomfortable, since we've been conditioned by decades of books about robots and other humanoid-type machines into thinking that if it looks human there must be some kind of humanity in there somewhere. And, since Feet of Clay, PTerry has fostered this idea by 'humanizing' the golems in Feet of Clay and subsequent books.
And then, once we all feel that all goblems have intelligence and self-awareness and are thus capable of being free and self-actualized, he subverts this whole idea by forcing Ankh-Morpork (and us) with golems that probably can't be self aware of truly sentient or self-actualized. Essentially, they are the same as real cattle or horses--maybe even less so, since they seem to have no sense of independent action (other than crying out for company, which a dog might do as well). They're beasts of burden. Moist, Vetinari, and even Adora Belle recognize this, and treat them accordingly--as tools to do work, but to be treated with care, like pack animals.
J-I-B