Not entirely sure why she can be targeted, it serves no purpose but to annoy controller players.

I've never had this issue until I started tanking expert roulettes. I immediately have to swap from her to the nearest mob when I want to gap close onto that pack of mobs and when I pass her and I want to quickly target the mobs behind me so I can throw my enmity projective as I'm dragging them to the next pack(but of course I can't because she's right there, preventing me from getting those one or two extra hits in).