Like Frostbite said, the Kroot and the Tau have an arrangement.
The Tau helped the Kroot fight off an orcish invasion, and then made a deal with them, so the Kroot are, for all intents and purposes, part of the Tau Empire.
The Kroot, however, have a mercenary mentality, that's pretty much part of thier whole evolutionary process: They will fight for reward. That reward is usually secondary to them getting to sample new genetics for their whole wierd evolutionary processes.
That said, the Tau find the whole mercenary "fighting for reward" and not for The Greater Good thing to be completely alien. And if the tau discovered the Kroot are selling themselves out as mercenaries, they'd be very upset.
As a result, the Kroot take great effort to keep the tau from coming across these merc warbands.
Or so I've read.