Check out the Deathwatch RPG, the Tau like everyone else have a very dark shadow. Forced sterilization, oppression in the name of the 'Greater Good'. As they are written in the Codex the Tau don't 'fit' in the 40k lore, they are to 'good'. Remeber we have 20+ years of lore for how the Imperium has gotten where it is. They - like the tau - are pragmatic in promoting a greater good.
Brighter the light, the darker the shadow.
There are NO good guys in the 40k universe, just an unending sea of grey about to be consumed by Chaos and/or the Tyranids
We currently do not trust humans with our tech until the Imperium's total annexation into the Tau empire. Food and other necessities will be generously provided for those who follow the path of the Greater Good. However all other luxuries are limited to Tau citizens.