Yeah, I only call him out purely to be a dick.
He can deal with it, though, if he wants to keep taking my cash a Jackson at a time. He's got all the bragging rights he can throw in my face on social media (and he does).
I'm actually really confused here.
This isn't a sanctioned round. The cash you are playing for is a personal bet, not a tournament purse. The rules you are playing under are part of the agreement of the bet (whether you realize it or not). So, you can easily remedy this by simply saying "If you want to play for money today, we have to play by official PDGA rules".
Of course, you can't quite do that, as you don't have the required number of players on the card, nor a TD to decide any rules questions. However, you can also come to agreement on these issues. If he would rather play this as a casual round, with practice throws, etc, you don't have to bet him.
(Side note, aren't you in the UK? Or am I misremembering? It's weird to see you refer to a "Jackson" as a monetary unit, if you are. That's a specific reference to the American president Andrew Jackson, who is currently on the $20. It's like me saying I was betting a
Loonie on something.)
As to whether practice putting/revenge putting is an advantage, well, he clearly actually feels it is. He clearly wants to keep taking the putts so much that he goes against your expressed wishes, therefore, he gains something out of it. It could be that it simply makes him feel better, and effects his mentality. That's an advantage. If if it just annoys you, and makes no difference to his play, that's still an advantage.
As to why the rule is in place, you don't need only one reason for a rule to exist. The simplest of all reasons is simply pace of play. The more people take practice throws, the greater the net effect on pace of play. Plus scoring shenanigans and disagreements. Plus the aforementioned advantages.