Such a camera would have to communicate wirelessly with the handheld/headset, though, which would introduce latency to the head tracking, and if there's one golden rule of VR, it's that you want your head tracking latency to be as low as humanly possible.
Edit:
They do, but as noted above, an external camera would need to communicate wirelessly with the NX, adding unavoidable latency. The only internal camera solution which might be able to provide positional tracking would be a TTF depth-sensing camera, but I don't believe they have the accuracy required thus far. One option would be to go with Vive-style lighthouses, which gives very accurate positional tracking which can be calculated entirely within the NX (the lighthouses are just passively shooting out lasers). This would probably be pretty expensive, though, and would require people to physically install two lighthouse-like devices on the walls of whatever room they would want to use NX VR in. Not something I'd expect from Nintendo.
It would definitely be interesting to see what kind of path Nintendo would go with VR, but I don't expect to find out any time soon.