Because that mentions "machine guns" but I don't see either "assault rifles" or "assault weapons" mentioned.
Both ATF regulations and US federal law define "machine gun" in such a way that it includes "assault rifles":
26 U.S. Code § 5845 - Definitions
(b) Machinegun
The term “machinegun” means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.
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Under this definition, the M-16, AK-74, and any other "assault rifle" is a "machinegun", because all of them have an automatic fire mode: multiple rounds on one trigger pull. So are automatic battle rifles like the M-14, AK-47, Valmet, etc., and submachine guns like the Thompson, M3, Sten, MP5, etc.
However, this definition does
not include what politicians have been calling "assault weapons," because those are only semi-automatic.