Assault rifles, by definition, have select fire capabilities. Commercially available civilian AR-15 is semi-auto single fire only. People are often confused by the AR designation, but that stands for Armalite, the original manufacturer of the rifles. They are officially called "assault-style" rifles, although that term isn't very popular because it seems like a minor quibble. In all other measures (shorter rifle, intermediate cartridge, detachable box magazine, range of 300 meters) the AR-15 meets the criteria to be called an "assault rifle," except for the select-fire.

It's worth mentioning that many popular models can be easily modified by a competent gunsmith to add burst and/or full auto firing. It's illegal, but that doesn't stop a terrorist who thinks they are going to need their rifle to join the insurrection. At that point it would be an assault rifle.