They ARE eating dead cells (although they are likely to be hoovering up dead neutrophils on the whole rather than cortical tissue). Infection is destroying his cortex bit by bit and the maggots are enjoying it.
Surprisingly, the Snopes.com informant went on to lend at least partial credence to the tale told in one of the email texts, namely the "ingrown hair" story, which claims the patient was treated at "Stanford ER." According to this source, the photographs in question were indeed taken at Stanford University Hospital, where the cancer patient was brought by ambulance after a minor traffic accident (note that he did not "walk in," complaining of feeling "a little wobbly on his feet," as the email alleges). The patient hadn't sought treatment for the disease, apparently -- even though it had progressed to the horrific point we see in the pictures -- "because the condition was not causing him pain."
None of the foregoing, obviously, was connected in any way with symptoms of an ingrown hair, but it would appear that whoever made up that fanciful version of events was at least dimly aware of the actual circumstances.