As others have suggested, it's because glass and ebony are so valuable, that they are willing to risk getting the Blight by working there.
Keep in mind that the Ghostgate is meant to keep the Blight out, but anyone can freely enter.
Perhaps somebody should make a mod that gives most NPCs their own dialogue.
That's what the LGNPC Series does. Although they haven't done any of the NPCs in the caves/mines yet.
I thought everyone knew that the only person that creatures attack is the Nerevarine. All others can do their daily tasks unmolested.
Haha, yes.