This is one of the most inane threads I have seen.
On a NEW tractor the manufacturer/importer MUST equip the tractor with a ROPS if the tractor is larger than a garden tractor. (basically everything from a SUB-Cut and larger, that is sold new, must have a ROPS or certified Cab installed) MANDATORY.
However some people remove those factory installed ROPS and for PERSONAL use that is legal. Stupid, but legal. VOLUNTARY.
For COMMERCIAL use, any tractor that WAS ORIGINALLY EQUIPPED with a ROPS or certified cab structure MUST be operated with the ROPS installed and upright according to OSHA regulations. So for commercial purposes it is illegal to remove or defeat it (which means that folding ROPS must be locked in the upright position during use, using factory spec bolts, pins, etc. MANDATORY.