How, exactly, is a dumb charger supposed to tell the iPhone that it can supply *more* than 500mA if not for the voltage dividers? And yes, it won't fry the port if it draws too much, but if the port shuts down power due to overcurrent, it won't charge either. I'm somewhat dubious, too, about a $5 wall wart from Fry's having overcurrent protection. I'm not sure what the iDevices do if you plug them into a real USB host interface that can actually talk. My guess is that they will honor whatever power spec the port claims (my Dell Mini 9 Hackintosh seems to only supply 300mA if you're on battery).
But for dumb chargers, it looks for the resistors. If it doesn't see the "Apple resistor values", it charges at the spec'd 500mA. If it does see the Apple special, it charges at more current. Apparently, if it sees *no* resistors, it doesn't charge at all. But why should Apple support an out of spec USB charger? 2 15k resistors aren't exactly going to cost you much money.
And by the way, my iPhone is currently charging on a $5 thing I got at Fry's the last time I was in CA. It most decidedly isn't certified by Apple so I don't know where you're getting your statement about having to buy an Apple approved charger.