This has been a big problem and evidence that people who are buying the iPhone are obviously not the tech-savvy types who typically have smartphones. People who usually have these types of devices know what can happen. I'm sure there is something in the material people are given that states there are constant data transfers when email is on, but people are too lazy and never read all the material when they get something new.
I worked in hotels for 3 years. Everyone knows that hotel long distance charges are ridiculously high and the rates are very plainly stated right on the phone, but we would always get people screaming at us over calls they had made and didn't want to pay for or out and out lie about even making the calls. I had one guy claim he didn't make a call, but admitted the number was his home phone number!! What, did someone else call your home? The housekeeper?
I think the first guy in the story should incur at least some of the cost. The second guy shouldn't have to incur any cost since he was using WiFi. If the device still connects to cell networks even when on WiFi, Apple/AT&T needs to fix it.