Pre-load of suspension forks on one side only
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.