The reason, as I know it, why thieves/assassins lose hide when they join the Outlanders is because hide is inherently a civilized skill. If you play a thief you would notice that thieves can mostly only hide in civilized areas. Not places that are considered wilderness. Civilized = cities. Outlanders are against cities. This is why they gain camouflage instead.

This is different from rangers and druids since even though they lose confidence in certain skills (such as camo) when they spend more time in city areas they may not necessarily be against civilization and cities like the Outlanders. This is why rangers can join the Tribunal if they're lawful. Being a ranger could just mean you're familiar with how to get by in the wild. Doesn't mean you have to be an ecologist or tree-hugger.

Someone please feel free to correct me if I'm wrong with any of this.

Edited: Changed it so it reads that 'they may not necessarily be against civilization' instead of 'they are not against civilization'.