If I buy an iPhone 5 from ATT and unlock it, even if I pay for a Straight Talk SIM and use that from time to time, I'm still on the hook to pay AT&T whatever monthly fee the contract obligates me to pay.
The only reason AT&T would want me to not unlock my phone, is to ensure they have a captive market for further services that are not part of the contract. Be they usage overages, international roaming, etc.
They'd like my $70/mo for a base smartphone package (with an absurd early termination fee to boot) and to not have to compete for any additional or international service I may need.
Now you can certainly argue that that's a consideration people are willingly agreeing to when they agree to buy subsidized phones on contract. But the argument is that it's a consideration that ought to be explicitly spelled out in the contract terms, rather than enforced by an end-run through copyright law that most people don't know about and have no reason to believe holds any bearing over how they use a device they're paying for.