How to get the unlocked HTC One onto AT&T for just $175

From this comment by gtg465x:

Buy an iPhone for $200 from AT&T, sell it unopened for $600 ($50 less than Apple sells it for), buy an unlocked HTC One for $575 (no tax and free shipping!), and you just got yourself an unlocked, unbranded One for $175, almost $50 cheaper than you would end up paying for AT&T’s crapware laden version after tax.

This sounds like a pretty good plan. I'd try it if I weren't on Verizon without a subsidized upgrade in sight (grandfathered unlimited data plan). But can anyone see any reason why this wouldn't work on AT&T?