Don't know if it was intentional, but this maligns some of the hardest-working and down-to-earth people that I've ever known as crooked.
But perhaps this is just referring to taxes, but farmers are also not incented to fudge too much on their taxes: they receive subsidies, and very little is paid with cash anymore, so is easily tracked.
Sadly, most farms today are huge corporate operations, so perhaps that's what you're talking about, but out of the less than 25% of the remaining family-sized operations, most are not wealthy by any stretch. Perhaps that's what they're doing "wrong".
I think one of the reasons you can't really be crooked as a farmer is because it's a small community and everyone knows everyone else. If you rip someone else off, everyone will know. It's much easier to be crooked in a bigger city where there's another sucker born every day.