You're missing the point. Nintendo could have forced that $10 on everyone if it wanted. It could have found a way to get that money one way or another. If not the AC adapter, it could have done so by increasing the price or cutting something else. The AC adapter is just something tangible that people can latch onto. "I don't want to pay for something I need to use the system." Fair. At the same time, I could claim, as a DSi owner with a perfectly functional AC adapter, that I don't want to pay for something I already have.
I'm not even defending Nintendo's choice not to include the AC adapter. I think it should be included out of principle, but I don't really see the cost being the problem. That's specifically what I disagree with. These companies make a killing on all kinds of things that we openly accept. We've dealt with this stuff before. The vast majority of Gamecube games required a memory card (unless you didn't want to save or advance) yet nobody really complained that one wasn't included with the console.
And $209.99 would be an unorthodox MSRP, but is it really that different from some of the others we've seen? The original Gameboy launched at $89.95. That's a pretty arbitrary number. 2DS launched at $129.99.