That's some really arid land out there, and the vegetation needs all the help it can get.
It's simpler than that, even. The tortoises love cow poop. The digestive system of cattle is not terribly efficient, so a lot of the nutrients from the grass is left behind, and the tortoises have been thriving on cow patties.
This zero-sum business of fewer cattle = more tortoises is, well, bullcrap.
But just try to tell that to a know-nothing enviroweenie.