I'm keeping my receipt just in case they try to slap me with a $500,000 fine (I can't remember the exact amount of the fine, but it's ridiculously steep) and throw me in jail for 10 years for unlocking my phone.

Seriously, though, the people unlocking your phone couldn't care less about when you bought it. The customer is the one who is liable, not them.
The most absurd thing about this is the only reason it's locked is because AT&T subsidized it and they need to recoup their initial loss. This is done slowly over time through our monthly bill. If we don't fulfill our contract (possibly because we unlock it and move to a different carrier), we have to pay them a hefty ETF. This covers the subsidization we initially received. So it's win-win for them. They will get their money one way or another. So why the threat of ten years jail-time and hundreds of thousands of dollars in fines? I doubt they will ever actually prosecute anyone for this, but it is a law nonetheless.
Verizon customers don't need to worry about it since their iPhone 5s are sold to them unlocked.