Pre-load of suspension forks on one side only I have a question about suspension forks that have a pre-load knob only on one side. 
I believe having it on one side only is not uncommon, and the other side is often equipped with a lock-out knob. 
My question is: why is there a pre-load knob only on one side. Particularly for rather simple (low-end) coil suspension forks having only one knob seems to imply that you are pre-loading only one of the two coils in the fork, while the other spring would remain unchanged.

1) Is this assumption (that only one of the two coils gets pre-loaded) correct?

2) Why is it done like this? (Would this be done to have the "sag distance" still be covered by the other coil?)

3) Doesn't pre-loading only one of the two coils cause an unfavorable asymmetry?
1: Your assumption is correct yes, only one side gets "preloaded".  

2:It's done like that mostly to save on expense and because a preload adjustment on both sides would be overly fiddly when the adjustment can be accomplished with one knob.  

3: Asymmetry, yes.  Unfavorable, not so much.  Forks are designed to be very stiff where they are not supposed to be suspending.  That being the case, the two sides should always move as a single unit, thus only one preload is necessary because ultimately the forks preload is going to be an average between the preload of each side.  That means expense and effort can be saved by putting adjustments on only one side that will ultimately affect the fork as a whole.

As you can see from the shot below, the internals on a fork may be very different by side.  Suspension forks are designed with those stiff arches allow each side to move only as a unit.