While I agree that there is some responsibility with the customer, the telco still has some responsibility. Were are at the beginning of a new technology "age", many people just don't understand most of the finer points. You can't see a megabyte, and it's hard to see data being used. The only party who knows for certain how much data you've used is the telco.
Notifications are sent out by SMS to warn you that you are low on plan and/or pack data. Also, balance requests on 2degrees are accurate to within 10MB (which is the most common system reservation size) even for postpay, due to our use of real-time charging for all subscribers. This information is accessible either from the self-care web site (which is free to access), or via USSD. We used to send out SMS notifications when the data was completely used up too, but we turned it off because subscribers complained about the spam. (Can't win either way!)
And don't forget spend control is always there.