From what I've garnered (one of my brothers plays tau, the other, ultrasmurfs, they both found fluffy reasons to ally against ork and chaos, but also have plenty of excuses to railgun/lascannon each other), the tau is regarded as a sovereign empire outside the Imperium's grasp. As such, the Imperium at this time cannot and will not spare the military force to smite their small empire from the galaxy. If they did, their primary objectives of holding back Chaos, purging heretics and fighting off the necrons, tyranids, orks, eldar and whatever else the tau aren't, would suffer for their abandoning the fight, if you will.
The Imperium isn't the Tau's ally, nor is the Tau the Imperium's, but the Tau can't traverse the warp, except in short leaps and skips, they don't have navigators, as they don't have sensitivity to the warp, so the Imperium doesn't regard them as a threat, but knows they're too well armed to invade and crush.
The Tau don't want Imperium allies, they want human vassal worlds. It is these worlds which the Imperium fights hardest for, as they do not wish to lose tithing worlds, and they will not tolerate xeno-friendly collaborators.
When the Imperium knows a world is lost (tomb worlds, those lost to hive fleets, etc) they'll virus bomb it, and because the Tau do follow a sense of honor, the Imperium tends to give them a heads up. Not all chapters are as noble as the Imperial Fists and Ultramarines though. I'm sure some chapters would rather just leave the world, virus bomb it and then assault the tau warships in orbit.
