First off I'd like to say that I'm not saying the Tau are perfect and have no flaws and are the nicest guys ever. Like everyone else, they are out to rule the galaxy.
I have seen many times people say the Tau have a 'join or die' mentality, or that something is controlling the Tau and they are therefore just slaves to a higher purpose or the Ethereals, or that they have no free-will as such (e.g being 'forced' into a caste and not being allowed to do what they want to do)
There has been no time that i have seen (in the 8 years since i started W40K) that suggests this at all. They didn't kill the Vespid when they refused, and several other races are only trading with them instead of being an actual part of the Empire. The only things i can think of that even slightly suggests this would be things such as the Dawn of War games, but in those times it is someone who is already their enemy refusing to join.
Nothing suggests there is anything controlling the Tau (such as pheremones) other than random Imperial Speculation because they refuse to believe the Tau would just obey.
The Tau are the most 'good' out of all the races from what i can see. Can anyone explain if i am missing something?