Fascinating question...
Short answer = don't know ... but I would think the reason for the restriction may be that circular section (round) wood may begin to spin on its long axis as the saw teeth begin to bite - hand holding wood quickly gets hot - operator lets go and workpiece flys about out of control striking operator senseless... Just a guess...
I would take a chance (being senseless) but would make sure that the workpiece is firmly held and can't spin.
I can't see the saw blade being at all bothered by the cross section of the wood it's cutting...
Peccavi
December 2008