They are not. What you call an assault rifle, the AR-15 is not. It's merely a semi-auto rifle like a hundred other models. Any type of gun or other items can be used to assault someone, e.g., an assault weapon: even a knife or hammer which are used hundreds of times a year to assault a person.
The irony of their argument is that they never include machine guns which are owned, legally, by over a million citizens.