There seems to be a vast difference in quality of the wood and treatment between the products sold in the DIY sheds, agricultural suppliers and that used for items such as power and telegraph poles.
I note that where I'm currently visiting the fences that are used by the nearby farm are held up with 6 inch round posts and are from complete 7 inch tree trunks that have only been de-barked, cut to length and have a point at the ground end. They don't seem to have been treated.
Power and telegraph poles are probably similar in that the are not cut from larger timer but are whole de-barked tree trunks and probably everything below ground was covered in a thick layer of bitumen.