The M16, which the AR-15 resembles, and A4 are not used exclusively to conduct assaults. Mostly this mischaracterization is due to fear and no understanding of weapons or ballistics, or it comes from a misguided definition of an assault rifle put out by the U.S.
The fact is, it isn’t a military weapon at all, and cannot be used during U.S. Rhetoric claiming that an AR-15 is an assault rifle is an ignorant mischaracterization of the weapon.