People backed the project before knowing how the game is going to be, before reading reviews, before playing demos... even if 15 bucks is a good price, it's not the same that when you buy a game in the usual way. I'm not saying that DF owes us a free engine, not at all. They never said they were going to release it.
But I think that this its a great oportunity to explore new ways of game development, something different. DF doesn't have to bargain with a publisher, they are free to do whatever they want. Now its time to consider if releasing the engine woulb be something good for DF, would it help? Well, I think so.
If a free engine is born out of this, and people start using it, will it make DF sell less games? If other adventures are made with this engine, will that prevent DF of selling their own? I don't know, but I don't think that would happen. And, there are positives? Will people implement new features? Will the code be maintain? Ported to other systems? I think that could happen, and DF would benefice of it.
I don't think that releasing the engine would prevent DF from doing a few more adventures with it. I think it would be a good thing, for DF and adventure fans.