Just a guess and speculation on my part, but:
They could've added the necessary "X" to help out the casual gamers. They may have had some complaints or requests that casual gamers were changing assignments without their knowing and they couldn't figure it out.
IMO, they added one extra step because they figured people who wouldn't use it or know how to would be susceptible to an accidental press of an adjustment button, while they assumed adding one step in between the adjustments wouldn't hurt the more hardcore gamers who knew how and when to use the playmaker.