So what is it about pith that makes turning anything with
pith in it a No No!?

Not all tree have pith. What they are saying is that a green piece of wood that
still contains the center of the log is more likely to check due to internal
stresses caused by drying shrinkage. The forces are greater at the periphery,
and are nil at the center. These differential forces are the culprit. Such a
log will check at the outer edge and it may continue inward towards the center.
An engineer would say the center is the 'neutral axis'. Remove the neutral axis
and the shrinkage forces have less to work on. Sorta like removing the fulcrum
from under a lever.

Whenever I process a tree, I have my lumberjack cut down the middle of the log
with his portable sawmill. On a large log, he'll take out a 2" slab from the
center.

Dan