She does some shady shit.
If you call her out or annex the forsaken, you risk losing the bloodelves. So then we have 3 warring factions. Horde - BE+Forsaken - Alliance. That seems like a stupid move yes?
Does it not make much more sense to allow her to remain since she is (for the most part) doing what you want her to. Granted you always watch your back and you sleep with one eye open, but I think any leader would be stupid not to in the fantasy world.