I don't think Amazon has that much of a problem with the jailbreaks. I can't see how they would loose any money with it, and who uses them, knows that he can potentially brick the device.
As far as ad-supported services go, the Kindle is actually quite good - totally unobtrusive, and you actually get some good offers, things like $5 off a $10 purchase.
In fact, I wasn't even really able to find anything other than removing the ads and changing the screensaver as what you can do after your kindle is jailbroken.
After the initial effort investment (not that hard, since you can download it with the kindle's browser), I would say it's actually easier.
It's only a few more seconds than an Amazon buy.