Not sure what you mean, "by law." As I understand it, when my tractor was made in 1969 there was no requirement for rollover protection, not even seat belts, and when the laws were introduced, all existing tractors were 'grandfathered in.'
There is no law requiring me to have rollover protection on my 44 year old tractor. That said, it's a good idea. So, in your pictures there was a 135 with a rollbar. is this a custom creation or a factory modification?