It's actually a combination of the two, but more AT&T than Apple. When AT&T were setting up the plans for the iPhone, they - being the experienced wireless provider - should've said "Whoa there! If this thing is going to be pinging the network for data every minute of every day then it's going to be hell for anyone taking outside of the data plan area. We should sell a decent international plan with this or give them a way to shut down the data connection."
Apple didn't think of it, which is poor but they aren't wireless providers.
AT&T didn't think of it or didn't care. If they had half a brain, they would've called up all their existing customers who have an international calling plan (like me) and who'd just switched to the iPhone, and sold them the crappy international plan they currently offer. But it's hard to criticise them for not selling it correctly at the point of sale, because it was all activated online. Though iTunes...