You're right, I could rack up much more than that if I really tried but I've never been one to watch movies on mobile. It mostly consists of a few news apps and the occasional tethering for quick job-related tasks.
I can see how this plan might not work for everyone but as an American who rarely travels internationally this is fine. And if I tried my best I could probably only consume 10-15GB a month so it's essentially unlimited to me.