Stuff like this happens all the time with software projects. The thing is that we never saw it happening before our eyes because publishers simply said to the press "release date for game X is delayed to date Y" and everyone accepts that. But no one really knows why the delay. In such a situation a developer would go to the publisher to ask for more money and more time, and the publisher could just cancel the project, force a reduction of scope or quality, or believe in the developer and give the extensions. Again, we never saw this happening before because it was "behind walls", but with Kickstarter wee see all this.
Yes, this was clearly a planning mistake from Double Fine and Tim Shafer. But these things happen even to experienced devs. This just serves to remind people that Kickstarter is always a risk.
I like what I've saw so far in the development videos. They have provided me with a lot more entertainment than my pledge money is worth. Of course I'm not thrilled with the news but I don't think it is necessary to be overly harsh with Double Fine.
EDIT: Oh, and the "they got 8x what they asked for!" complaint is not entirely reasonable. The game's scope and production also increased a lot from the original idea. The original pitch was for a simple and small adventure game, something that could be done with 300k (100k was for the documentary). The game they're developing now is much, much bigger, and with higher production values (more/better art, voice acting, etc). As I said it was a planning mistake and they ended up with a much more ambitious game than what was possible with 3M, but if they just delivered the originally-planned 300k simple adventure a lot of people would be pissed. It's hard to manage expectations and project scope when funding is so much bigger than expected.