If those items always required the extra purchase of batteries, then the expectation is set that power supply isn't included.
???
Consumer electronics are never consistent. It depends on the maker, the item being sold, and so on. Whenever you buy electronics, you should check the box if it includes batteries or not if it needs them.
Monitors regularly come with unusable VGA cables for most modern computers. Have to check that every time you buy something like that. DSLR cameras regularly come without a memory stick/card, making the camera useless until you acquire one. I can go into my lab right now, look at various consumer electronics we buy (because mark-ups from specialized retailers are stupid), and many of them come in a way that will not work out of the box without further purchases.
Heck, we recently purchased a Canon DSLR and the thing won't do anything unless you give it an SD card. This is a thousand dollar purchase... and it doesn't work without an additional purchase.
Nintendo handhelds haven't required purchasing power supply in addition to the hardware since the GBA here in the US. The expectation had been set that paying for a device gets you a charger, so no additional purchase is required. It was considered complete.
The company is now altering 12 years of history, just to save a couple of dollars per unit. Something that consumers used to get as part of their purchase is gone. That-- at least to some-- gives the sense of an incomplete package.
Its shitty but you seem to have a misconception of how widespread this sort of thing is... but this is a revision and last time around they just arbitrarily changed the adapter. Its better but its also stupid. But when it comes to actual fresh hardware, they're not going to ship you a unit without a charger.
Also I have to wonder how much it actually saves them overall because I think its more than a couple of dollars if we calculate the price of the adapter + the decreased cost per volume for shipping. The N3DS box is much, much smaller than the old 3DS box. There's also shelf-space costs which are also further cut down by the smaller footprint.
They could have included a USB cable, though, for space reasons.