Because it was there
This reminds me of the days of PSone and Xbox hacks. Lots of folks went around reverse engineering the boxen, building and installing mod-chips, etc. Hey, it's not an Apple specific thing...really, what this is about is human curiosity and arbitrage.
If you have a black box, and you can play with it and get some secrets out of it, you're already a rockstar. Publish your work, and any code monkey, theoretically, can replicate your work. You've just employed a million eBay drones. Also, there are markets where Apple is barred from entering. Guess who's selling iPhones into these markets? Apple doesn't care, as long as they get the revenue from the individual phone. If the iPhone were being cloned at the hardware level, then they'd be pissed. Extra revenue from carriers is just icing on the cake (and good on them for pulling that out of AT&T).
The reason a phone can be carrier locked, in the U.S., is that there is no national standard for cellular access. Here, we have 2: CDMA and GSM. GSM also works in Europe, while CDMA is strongest in South Korea. This is a problem in the States because the two biggest carriers are AT&T (GSM), and Verizon (CDMA). Now, in order to satisfy carrier neutrality rules, you'd have to stick 2 antennas in the phone (or use a software-defined radio, but no, we're not going there, yet). Because the FCC is a bunch of cowards, there are no carrier neutrality rules, therefore, AT&T and Apple are allowed to have this cozy relationship.
It doesn't matter, though, as long as Apple is seen as relatively good, AT&T is seen as all evil, and the hacker scene is left to carve out its niche, things will go smoothly. If people wake up to the fact that Apple is quietly raking in a ton of dough, the citizens of this great nation may finally put a stop to forced vendor lock on handsets (yeah, right, like that's ever going to happen). Bottom Line: Apple does not care about AT&T's bottom line. If people choose to hack their iPhones let them.
I'm waiting for a fully functional N96 to cross the pond, so I can play with it. Until then, "old" Samsung for me.