First off, the adapter is simplicity in and of itself. You plug it in, turn on the computer, a program runs, you turn on your DS, and you're online. I don't think it gets much simpler than that. All anyone needs prior is to have a DSL/cable connection. WEP and security are handled with ease. I've worked in tech support and I know how amazingly stupid people are, but Nintendo has removed as many hurdles short of them putting it into the usb drive themselves.
Secondly, "free" applies to service charges (Xbox Live). I realize what you are getting at - that an adapter looks like it eliminates the "free" mentality - but this is a situation where semantics get in the way and people suddenly start thinking "free" also means they'll get a lot more than is being offered. Nintendo can't be blamed for that.
Third, the problem with an online only release is as I've pointed out already - about 2-3% of us might know about it. Of that, I'm sure the majority have wireless networks in place already, meaning they won't buy it UNLESS they feel the need to support Nintendo (something I intend to do, for example). And the problem with that is that Nintendo is going to come back in six months and say "Oh well the adapter didn't sell well so we're dropping it, obviously less than 1% of DS users go online and don't want wifi," and retreat back into their shells where they refuse to adapt new technologies from fear that it is so "totally unprofitable, man."
(YES I KNOW THE REV IS NEW TECHNOLOGY, I am speaking in terms of CDs, online, etc. The same shit they've always pulled and only hurts them down the line.)
The same thing happened with the component cables for the Gamecube to the point where Nintendo took the damn ports off the Gamcube itself, a move that caused a huge backlash among the gaming community and still does to some extent. Granted it's not as huge as, say, 360's backwards compatability issues, but it is still enough to add to a growing list of concerns that Nintendo is harboring right now.
Here's the solution. You release Mario Kart. People go buy it. It is advertised as wifi capable. You put up a big sign that says ASK ME ABOUT WIRELESS at stores. Then you send out Nintendo reps to a wide range of stores who TELL CONSUMERS ABOUT WIFI, making SURE they understand that DS can work with all wireless networks now, and that the adapter is only needed if someone hasn't set theirs up yet. You do that for the first month or so of the big wifi push. THEN, you release the adapter in limited quantities to stores, and instruct those same stores of the same information the reps had. Additionally you make the adapter stocked in glass cases behind the counter instead of out in the open, so it minimizes the number of people picking it up because they think it is required.
You put this information all over the place - posters, commercials on tv/movies, billboards, signs, magazines, instruction manuals - EVERY CONCIEVABLE PLACE.
I realize that Nintendo can't monitor everything, but they have commercials, reps, and the ability to educate people as best they can. They can meet people 80% of the way through and hope that people understand the final twenty.
I understand the whole non-confusion thing, but I think there are ways around that, where you blitz the market with what could be your biggest and most profitable DS game yet, the one that's going to jumpstart your wireless online campaign and sell a LOT of units. I think the offset to that is you get your ass in gear and not cripple it at the beginning because you still want to sit back and see what happens. I hope they have 10K units on their site and get orders for 500K, showing them that they are vastly underrating the need for a USB adapter that takes all the guessing out of the picture.
But I guess their service hotline is going to get slammed. "I CAN'T CONNECT TO MY NETWORK TO PLAY MARIO KART!" and it's going to be because of security issues, beacuse of bridge setting problems, and a thousand and one other things that could all be easily remedied by a simple adapter that probably cost them $5 to manufacture.