I agree that there are various agencies involved. The standards are established, there is oversight of units being sold in US and OSHA and Dept of Labor keep track of the workplace. I believe I can do what I like with my tractor and no one is telling me I need a ROPS on my Farmall A, but if I were employing others the oversight is different.
Loren
It is my understanding that you can do what you want with it, after you buy it, if it is for personal use.
But, I have seen posts where owners have been advised by dealers they cannot sell used equipment that was originally manufactured with a ROPS, without the original ROPS installed.
I do not know if it is true, but from a liability stand point, it would make sense.