While it is easy to get upset by this – after all, a software mod shouldn't void a hardware warranty – that's still Apple's policy and anyone who does tinker with their phone is likely aware of the risk. The blog mentions a very good point that anyone fiddling with the inner workings of their iPhone would do well to remember: reset to defaults if you need to send it back:
So while he eventually got Apple to return the iPhone (after paying the restocking fee, that is), the lesson here is to do a restore of your iPhone and re-install the factory AT&T SIM before bringing into an Apple Store for service. That’s the beauty of software-only hacks, after all!
Truly, you cannot criticize Apple for taking this stance. They sell the iPhone as a hardware and software bundle, and don't intend for it to be taken apart. With modding there is always a risk for the manufacturer to give you the cold shoulder when it comes to an RMA.