I'm not much of a MTBer. I really know very little about it. But I know that when I do ride "off road" (usually hilly dirt paths with a couple rocks and roots. No DH or jumps or anything), I like to have multiple gears because lets face it, riding on dirt/mud is tougher than asphalt.

So why do companies all have 29er MTB's with single speed set-ups? Are these actually meant for anyone to take off-road? Do people ride trails with SS?
Or are these bikes meant for the "I want a MTB shaped hybrid bike" crowd? You know, the kind of people who buy a Giant Boulder so that they can ride with the kids one Sunday every summer.

I'm actually asking the question honestly.

I'm editing to add:
The only real use I could see for a SS 29er with disk brakes is if you are a bike commuter (which I am) and if you had a very flat commute but with lots of potholes and whatnot. So you'd get the ruggedness of the MTB frame, the shock absorption and grip of 29ers, and you don't need more than one gear since you don't have hills. But that's really the only practical use I can think of.