Karait, I think your "conclusion" that the standard applies to all powered industrial trucks is mistaken. Your quoted ISO standard says clearly that it applies only to "Devices for towing" and clearly states it covers; "Trucks used for towing trailers". You are using a standard that covers the safety of the hitch, not that ALL trucks can have a hitch.
No one can accurately define what weight a trailer can be loaded with, to reflect 'draw bar pull', since raw weight is only 1 of the factors of 'draw bar pull'.
IMNSHO; a far more accurate 'conclusion' of that section of the ISO standard would be that the ISO recognizes that some times a powered industrial truck (which includes towing tractors as a 'class' of powered industrial truck) may be required to pull and when it is, it must be fitted with -proper- devices to attach it to what it is towing, not so much that ANY Powered Industrial Truck should expect to be used to tow.

Not all powered industrial trucks are tow tractors, but all tow tractors used indoors to primarily move goods and material are a type of powered industrial truck.