Well, isn't this surprising, it looks like once again the carriers are locking down a Samsung device. Of course, it isn't just Samsung devices that the carriers lock down but, in the case of the Galaxy S 4 there are going to be more people out there looking to get custom software up and running on their phones than other devices. Steve Kondik - who you might have heard of - discovered as much for the AT&T Galaxy S 4 and posted the following on his Google+ page:
"Yep, it's confirmed. The AT&T S4 authenticates the recovery and boot images before executing them."
You'd think that this were being done to keep your smartphone safe, right? Well...
"The only outcome I see here is stacks of bricked devices being sent back for warranty replacement due to the ease of causing a permanent boot failure, especially since the device is trivially rootable."
It appears that it's not just the AT&T Galaxy S 4 that has it's bootloader locked down. Brian Klug of AnAndTech hinted on Twitter that all US carrier variants of the Galaxy S 4 are locked down, or at least those shipping with Samsung's Knox security suite:
So, it could well be Samsung's own security software that's keeping that bootloader locked down, or maybe it's just the carriers being the carriers. It's more than likely going to be the carriers once again here and while you can certainly argue that there's a minor risk when unlocking your device's bootloader, there's no reason to lock things down like this. If we want to put custom software on our devices, than there should be nothing standing in our way. I think Mr Kondik says it very well himself:
"I can't see what AT&T has to possibly gain from this. GSM and LTE aren't magical, tethering is controllable on the server side, and theft-of-services is not possible from the application processor side (or even from the modem side as far as I know). The same device is available on every carrier, so it's not an exclusivity issue either. The modem processor has always been locked, and the casual user doesn't want to mess with that part anyway. Samsung has always been developer-friendly, so I am guessing their hand was forced."
Regardless of whether or not this is Samsung's doing or just more pushing around of consumers by the carriers, it isn't right and we hope that Samsung provide some sort of tool to unlock the bootloader (yeah....) or at least some enterprising devs find a way to crack this thing open like a walnut.