I assume it's because of the use of cartridge bearings instead of loose or retainer held balls. With a loose ball threaded headset, I agree that a very small adjustment makes a big difference in steering resistance and adjusting them properly takes a deft touch. Cartridge bearing headsets, both threaded and threadless, can accept, or even require, a bit of preload. Several threadless cartridge bearing headsets I've worked on (Chris King, FSA and Cane Creek) specify preloads of 8 to 20 inch-pounds.