The owners manual says to use ATF because it was written years and years ago. At that time ATF was a satisfactory hydraulic fluid with no ill effects. Since that time ATF has gone through drastic evolution. The ATF of today has additives that are BAD for the seals in your power steering system.
When your owners manual was printed, most all cars used ATF in the p/s systems so you didn't even see a different fluid on the shelf for power steering. It was not necessary.
Once the mfg's saw that ATF would wreck the p/s system then the "Power Steering Fluid" products started showing up on the shelves at the dealers as well as the parts stores. Unfortunately, they had no way of going back, pulling the owners manual from your glove compartment and rewriting it.
Hope this explains it,