Because the contract isn't the same as a bank loan. They're suppose to be making money on the voice/data usage for 2 years, but you're not using the AT&T sim card anymore. I thought the person I was replying to wasn't happy with the ETF fee since he was debating with the mikestew who said he just paid ETF to make the carrier switch.
If we're all in agreement that the current business model needs the ETF fee before unlocking, then I guess I have nothing to say. But anyone who wants their phone unlocked without paying ETF is missing the part where carriers would get screwed on high-end phones.
Get a $200 iphone5 from AT&T, next day unlock it without paying ETF and get a cheap prepaid T-Mobile sim and never use the AT&T sim again; thus not paying AT&T's voice/data fees(and we all know they get you on overages). That'd be a problem for the current business model. To make up for the lack of overage-charges they'd have to make the phones more expensive or raise the monthly minimum charge.
NOTE: I understand the awkward issue of travel though. I made the mistake of using my AT&T sim in Nigeria once. Maybe smartphones should accept difference SIMs after it determines its in a country other than where it was bought. But of course, then I go to amazon.co.uk and order an iPhone5 to be sent to America. ;)