Because in the civilian world there are so many AR-15 models available by companies like Bushmaster, Magpul etc. most people don't refer to their weapons as the specific name given to it by the manufacturer. Because whether or not, it is still an AR-15 model. Most people also generally customize weapons to their liking so they never stay a standard model anyways.
About the "There nothing "civilian" or "military" about a firearm"
That's bullshit, a military firearm is set to a STANDARD such as the M4A1, it is a standard military US issue firearm, based on the AR-15 platform. Every single person gets the same model, and might have different external accessories because of their different trade.
But with civilian weapons, they usually aren't select fire (Unless it's legal for you to own one) and especially with AR-15s they usually do not follow a set standard. Which is why, unless you VERY specifically want people to know you got a firearm from a certain company, you call it an AR-15 rather then something like an SR-16 or Marstar TNW.

That's wrong in so many minor ways that I can't be bother. Have fun with your miserable life. I refer to my SR16 as 'the SR16', cuz if i'd say the AR15 people would go "which one".

Guess ;)

NSFW? xD

You know it :D

Haha xD