Nintendo Direct / 3DS XL

[Mutsukki]

Including the adapter would effect the price by such a small amount I really don't see it as a convincing argument.

Not only will the damn thing probably not be in stores (much like the Wii's component cable), it also means sites like Amazon will be flooded with crappy ones, showing photos of the actual one, and then sending you a turd as you struggle through the listings to find THE actual adapter.
And if stores were going to sell 3DS adapters they're more likely to sell there own cheaper brand.

But okay, we're "informed gamers" we'll be ok but...

I seriously doubt there's going to be a big read sticker saying "You must buy the adapter separately" on the box. So parents who buy it for a kid's birthday are going to be pretty miffed when they can't use the damn thing because they expected a way of powering the thing up to be included in the box because it's the norm to include the thing that powers something in the box. Yes stores can say "Excuse me sir, but are you aware this product doesn't come with a power-supply?" but I can't see Amazon or any online retailer drawing much attention to the fact.

The PS3 slim uses the same power cable as the PS3. A lot of things use that same cable. I probably have a spare cable somewhere around the house. I still expect my products to come with everything I need to enjoy them (aka turn them on).

If you already have a 3DS, it's trade-in value has now been hurt because you need the adapter. Nintendo is just being a dick by not including it and that's all there is to it. It's a lot of inconvenience for the sake of them spending $6 to include the damn thing.

I like how you work at the manufacturing line and know all costs and stuff

As much it is a dick move, it's not totally unreasonable. I mean, this is the same charger as the DS Lite's. It's terrible for new owners yeah (and for people who will sell the earlier version to buy the XL), but it's not that surprising.