So, they actually do have a "please no overages" box somewhere, but it doesn't actually work? :-) How convenient!
Did you demand a refund or issue a chargeback, or did you deem it was still within the terms you've agreed to? (Or just not worth investigating/bothering with?)
Does their access device not offer any internal (but user-accessible) statistics on usage? As a system software engineer, I can tell you that it's entirely possible and relatively trivial to design something like that, and have a captive portal that would take over once you are within a certain percentage of usage, on which you could decide what to do next. Of course, whether or not you have something like that is defined by a business need. :-) On my unlocked Google Nexus, I know exactly how much data I've used in any given period, even without having to consult any on-network billing departments. On the other hand, I haven't heard of Apple implementing any service as rich and detailed yet.