Motorola's relationship with unlockable bootloaders, and thus with the ROM-flashing aftermarket community, can be summed up as "complicated." While its default approach is to offer a consumer-friendly bootloader unlock on most of its phones, it nonetheless bows to the whims of its carrier masters (Verizon and AT&T) whenever they insist that said feature be disabled, and they don't offer those handy full-price Developer Editions anymore, either. Add Amazon to the list, because the retailer's customized, super-cheap edition of the fourth-generation Moto G can't be bootloader unlocked.
So sayeth the Motorola customer support page here. The Prime Exclusive Moto G (4th. Gen) is not eligible for bootloader unlock codes from either Motorola (which has the infrastructure in place) or Amazon (which doesn't). As a first-party manufacturer, Amazon has the same approach to its long-running Kindle Fire tablets. But those machines run Amazon's customized Fire OS version of Android and aren't carrier unlocked, whereas the Moto G 4th gen sold on Amazon is a standard Android phone with Google services. Amazon's software suite, and the integrated Amazon advertising that makes it $50 cheaper than the retail version, are simply layered on top of Android 6.0.
That integrated advertising is probably why the bootloader lock is going to stay in place. There's no reason to give your customers a steep discount on a phone simply to have them flash the stock firmware as soon as they get it delivered via two-day shipping. It's possible that developers will find a way around the bootloader lock, but I can't imagine that the Prime Exclusive version of the Moto G will be a priority with the standard version already available for just $200.
- Motorola support