I think Carry2 is on to something here. If it's true that Tau are genetically engineered to BE their caste (I really don't know much about Tau) than this is iffy by modern human ethics (even then, not all would likely disagree), but in the grimn future of the 41. millenium, it's downright charitable.
Because while a worker in a chemical plant might dream of being Lord Governor, when he wakes up the next morning, he is still a chemical worker, and he knows that, in 15 year's time, his health will have detoriated such that he won't be able to work anymore. Because for 99% of the Imperium, social mobility only goes downwards, and hopes and dreams are a source of misery, which is why the Imperial dogma often discourages wishing for things outside of your station.
And if you'd ask the Tau what they thought of their lack of freedom, they'd probably ask you "What lack of freedom?"