There is apparently no U.S. law against unlocking cell phones. Last year, the Library of Congress specifically excluded cell-phone unlocking from coverage under the Digital Millennium Copyright Act.
"for the sole purpose of lawfully connecting to a wireless telephone communication network."
So the folks selling on ebay, couldn't claim that exemption, since their purpose is profit. Apple probably could go after them. Not saying they will, but other carriers have gone after unlockers/resellers, and gotten settlements.
Also the DMCA exemption has a 3 year run, and then must be renewed by the Library of Congress.
It's too damn expensive for me regardless of carriers. Maybe some day it will be cheaper / not on AT&T, and I'll check it out.