The counter-argument to this is what about the poor Kid who isn't tall enough - I think the policy is in place with the small child in mind too (and the extra FP for the adult is no big deal). Take a family of 4 for instance riding splash mountain... the too small kid's fastpass opportunity gets completely wasted and she sits there waiting while parent 1 and 2 each ride once and the taller kid rides twice. The small kid can't fastpass a ride on her own...so I guess you could use their fastpass for another big ride (let's say Space Mountain)...so now that child waits around some more while an adult rides that.
That's why you would book 2 fastpasses for Splash (adult & kid), and say 2 for magic carpets (adult & kid), and get the rider swap at splash so that both adults can ride that (and yes the big kid rides twice so that each parent gets an enjoyable experience with their child). If not for this procedure...my 3 year old would not use any of her fastpasses, basically her fastpass experience would consist of watching us use a FP+ on Big Thunder, Space Mountain, Splash Mountain...and then we'd waste her 3 FPs. The way described above, the adults and older child got ride all three of those rides, and the small child got to fastpass 3 rides (for example Magic Carpets, Speedway, and Pooh).
Basically, IMO the difference between fairness and abuse can change with some perspective.
EDIT: I will add that I acknowledge that in my example we turned 12 fastpass slots (3 per person) into 18 actual individual ride experiences. But in no way was it done maliciously, we just want the 3 year old to have a good time too.