Major carriers like Verizon and AT&T have always hated the idea that customers can use mobile devices in ways they never intended, especially when it means they have the ability to easily move to another carrier. Fortunately for them, the US government has come to their aid in the name of copyright: starting tomorrow, a ruling made by the Librarian of Congress last October comes into effect, making it illegal to unlock a carrier-subsidized phone or tablet without authorization.
The changes re-write an exemption to rules under our good friend the Digital Millennium Copyright Act, which would normally allow modification for non-infringing purposes. Meaning that after today, if you buy a new phone or tablet from a carrier and take the liberty of unlocking it by yourself, you're technically breaking the law. It's difficult to see exactly why this pertains to copyright, but the Librarian of Congress reasons that, "the record clearly demonstrates that there is a wide range of alternatives from which consumers may choose in order to obtain an unlocked wireless phone" — the most prominent of those alternatives of course being shelling out for an unsubsidized phone at full price.
For subsidized devices, the new rules effectively force you to remain dependent on your carrier
Granted, most US carriers have gotten better about unlocking the phones they sell you, and some high-end devices like the Nexus 4 and Verizon's iPhone 5 are now being sold unlocked out of the box. But for subsidized devices, the new rules effectively force you to remain dependent on your carrier by mandating adherence to any arbitrary waiting period it may have — quite a bit of salt on the fresh wound of being forced into a 2-year contract.
It's also worth noting that the ruling upheld protection for rooting and jailbreaking, and doesn't apply to phones purchased before tomorrow or used ("legacy") devices. So it's certainly possible that those seeking unlocked handsets will simply go to the second-hand market for their future needs. But after today, if you're coming to your carrier expecting cheap devices without getting tied down by federal law, you'd best look elsewhere.