The way I see it is, tree's are a bit like people, they have their own built in imune system that fights off attacks of diseases ie fungal, viral, physical etc. when a tree becomes very old, damaged or the envronment has changed, then the tree's imune system may become impaired and it can no longer protect it'self as well.
Re the untreated post it will start to rot quite quickly as it has no renewable barrier/s to fend off the the attacks of fungi etc that feed on the wood, only a residue that is left behind from when it was alive. Most but not all fungi feed on dead wood and feed mainly on the 'Lignum' content, the stuff that gave the tree/wood it's stregnth, that why you can easilt crush a piece of rotten wood in your hand. Other things like resins in the tree/wood will help protect it, western red cedar contains lots of it and is one of the most durable of all woods. Regards Tbird+