The devices you buy from US mobile carriers are almost always locked to a single network, and unlocking them has been a legal gray area for the last few years. Now Congress is finally taking action to remedy that. The Unlocking Consumer Choice and Wireless Competition Act has been passed by the Senate, matching a House bill passed in February. As you can imagine, consumer rights groups are pretty jazzed.
The US mobile market is weird in that almost all the phones floating around here are locked to one carrier or another. You can usually request an unlock code from the carrier if you are not under contract or still paying off the device, but Sprint is different. It claims it doesn't have any mechanism to unlock phones for use on other US carriers right now, but that's going to change next year.
After MoDaCo's recent report that HTC's Bootloader Unlock tool didn't work for AT&T's One X variant, The Verge reached out to the Taiwanese manufacturer, and received a reply which suggested that the device has "restrictions" which prevent its bootloader from being unlockable: