To cut costs, Nintendo is selling the AC adapter as a stand alone,
Yes, to also cut the costs of the consumer, so that the majority of people that already have a compatible adapter (including DS ones it seems) don't need to have to buy the same thing again, saving both sides money.
Although I understand why it is included in NA, going by the "complain first, think later" culture of America that'd just feel ripped off if they weren't forced to re-buy something that likely already own, like what's already been shown in this thread and even found itself to be important enough to make the title. >_>
If it was a 100% required item, why on earth would it not be sold together? Pretty sure Nintendo has a philosophy of making customers very happy and satisfied so that they come back later on, trusting Nintendo, instead of just trying to make a "quick buck" off of them. Probably one reason why they've existed for over(?) 100 years. (A few people might like to compare that to many other currently known companies. The length of life of a company isn't just a coincidence.)
Also, should be obvious again, but imagine what would happen if it had another circle pad. Devs would just make games dependant on it, completely alienating all other 3DS owners. Not smart. Having an optional attachment lets devs use it if they wish, perhaps making control a bit better for those that buy it, but while also making sure that they make the game playable on the 3DS as it is for everyone.
I think the fact that it comes with a 4GB SD card is pretty big too.