From my vantage point:
1. The 10% has been an industry standard agreement, but usually reserved for full time agreements -- beyond the agreed upon T and E base. This started changing in the 90s as Tiger-fueled purses took this compensation to amazing heights, leading to Ernie Els paying a flat salary for his caddie -- no more 10%. More of the top players followed, including Tiger.
2. It was common for some pros, in events that his regular jock couldn't be on the bag for a weekend, to use a local for assistance. It would be standard practice to agree on a rate for the weekend and an agreed upon bonus if a winner.
So what Kuchar did was not out of the norm for anyone. It was not a great deal in terms of a win bonus -- and it looks like his brethren thought Kuch was being cheap and should have offered more. I do not know the background of the guy he chose, but if we was like most international locals, he doesn't know the going rate and it probably sounded great.
Bottom line: Guy made a bad agreement and Kuch was probably short-sighted when it came to playing out what might happen if he did win. It has been settled now and that will be that.