One of the most unused, but nice features of the Kindle with Keyboard 3G is that it has web access. Web access on a Kindle is not easy. It is fairly slow, static, cannot handle modern web standards well.
But it is free, and it works almost anywhere in the world. My sister-in-law brought her kindle to Kenya last year and was able to send us messages via twitter from out in the middle of no where.
But there are a few people that have really tried to break the system. The Kindle is fairly hackable. A few people have even tethered their computers to their kindle to try to obtain free internet access.
So after four years of unlimited internet, Amazon has instituted a 50 MB per month cap. If you go over the 50 MB, then you will get a notice letting you know that you have 24 more hours to use the 3G and then after that you will be locked out of 3G for the rest of the month.
This seems like a reasonable response to the problem.
But really it is just a short term response. The Kindle Touch 3G (and most likely all future kindles) no longer allows for free 3G web surfing. You can only access Amazon over 3G.