Why does NASA's B-52 008 have a smoking engine in this photograph?
@Adam is right. The aircraft, NB-52B has smoke generator installed to assist in tracking.

A smoke generator has been installed on the left wing of the Boeing NB-52B Stratofortress mothership to assist in tracking the airplane.

Its use during the test of X-38 is detailed here:

Outside I could see the white trail from the smoke generator in the number-2 engine nacelle of the NB-52B approaching from the east
... I aimed my telephoto nearly directly up at the tip of the smoke trail.

As the same aircraft was used during the X-43 tests, it certainly served the same purpose.