Testing if it works/how much they lose
My assumption is that they are testing two things:
1. That the button works in all scenarios (speed up crops, new shop stall, new machine production slot, etc.)
2. How the bottom line is affected. If, say, 5% of diamond use is accidental, what percent will profits drop?
People are reporting that Use Diamond buttons have increased in size, causing them to repeatedly lose diamonds when listing in their shops. If that's the case, I think it would be even more important to have confirmation.
The excuse why there isn't already diamond use confirmation seems to have been 'it will frustrate users who intend to spend diamonds and slow down gameplay.' I never understood why it wasn't just made an option, like turning off music.
In any case, I've spent money on games in the past, but I have never bought diamonds in this game, even though I love the game. I just won't spend money on a game that relies on user error to generate profit.