You are underestimating the marketing value of the 3D effects. Of course the DVD sales will be great, because it has been on everyones radar for the last year+. 3D effects were a big part of that.
People saw AVATAR because hundreds of millions were spent on marketing, it had James Cameron's name attached, and it was well liked by popular critics and audiences. 3D was a footnote compared to that attention.
3D effects do no marketing favors for middling or outright-poor movies. The only thing they consistently do, is increase average ticket prices.
A friend sees the movie and says 'you HAVE to see this in 3D' and it becomes less about the plot, and more about the effects driving them to purchase a ticket.
You can't show the 3D in marketing, but you can (and they did) highlight the concept of it. It's like all those ads on TV about fancy TV screens with superior image quality -- they obviously don't show you the screen itself.
If it was 2D it would have likely sold the same number of tickets, just at a lower per-ticket price. Consider that it's DVD and Blu-Ray sales are commensurate with its box-office gross, with no 3D on offer.