Cost, also size and weight. If they used an AC induction motor it would be much larger, heavier and more expensive as well as the electronics would have to be much more complex as it would require a 3 phase VFD.
I suspect marketing BS is how they get 18A, it's not uncommon for unusual rating systems to be used with special purpose motors. For example my air compressor claims "5 horsepower" but it plugs into a 120V 15A receptacle.