In the US, VZW and AT&T prefer the bootloaders to be locked. One can speculate as to why, I have my theories, though I am not sure any carrier has specified why. If it is branded, it is likely going to be locked. This is not exclusive.
For carriers selling the international version, they may opt to lock them or not. I am not too familiar with them. The US usually gets different models, while for the most part, the European versions tend to share the same or similar model numbers. Often times, hardware is also different.
You can almost bet a VZW or AT&T phone will have a locked bootloader. That does not explicitly mean that root cannot be established, if I am not mistaken. However, IMO, it is preferred to be able to "fastboot oem unlock" than to have to use Odin or RSDLite a file to get the bootloader unlocked. I want the option there to do so when I please.
If an international, developer or GPE device is offered, I will pay cash for it. I only recently took a Moto X on contract with AT&T because I killed my phone. It is a great phone, but I take frequent nandroid backups of my phone and store them. Without a custom recovery, it isn't possible.
Sometimes, IMO, it isn't worth the hassle to buy a carrier branded phone. They also tend to receive updates significantly more slowly than the non-carrier branded versions because once it goes from Google to OEM, branded phones then have the carrier messing with settings.
/end
I think...I am mostly babbling in the attempt to show a different viewpoint which may not be applicable to all readers.
