They say that because “assault rifle” has a definition which the vast majority of citizen owned AR-15s do not meet.
Every time someone uses this term incorrectly, like now, it reinforces their perception that those opposed to gun ownership have no idea what they’re taking about regarding guns.
To avoid this, we should be willing to at least look up the simplest of definitions.
Semi-automatic-only rifles like the Colt AR-15 are not assault rifles; they do not have select-fire capabilities.
https://en.m.wikipedia.org/wiki/Assault_rifle
Then, usually, the response is “yeah well now we’re just splitting hairs/arguing about terms which doesn’t matter” to which I would respond “this thread started with arguing about terms”.