it's all about the headstock tilt- many tilted headstock designs don't have the string trees, take gibson for instance- Les Paul/ SG/ Explorer- they are all devoid of the string trees. the explorer has a 6-a side tuner arangement but still has no string trees.
straight headstock design often incorporate string trees because it's necessary to push down the strings to give added pressure to the nut, otherwise it'll affect tuning stability. fenders have 'em, Andy Timmon's Ibanez guitars have 'em too... all non-tilted headstocks...
FYI- recently, Fender's Am Series strats (WEF 1991) only sport one string tree instead of 2. the new staggered machine heads of the strat made sure that the ones further away from the nut have a steeper break angle to eliminate the need for a string tree. thusfar, it works for the G + D strings, the E + B strings still need a string tree fr added pressure. the next time you visit the guitar store, check out the machine heads of these strats, you'll notice 2 units to be taller than the remaining 4...