Ok, first, I want you to know that I'm not a Cop and neither "play one" on TV nor the web. I'm just a 50 year old Dad and ex-husband, and
I PROFILE NEARLY EVERYONE !!...and I notice things that are "out of the ordinary", be they people or situations... why? Because they ARE out of the ordinary and therefore stand out. You do it too; you do it all the time. Think not?
You see a man walking down the sidewalk in a winter jacket and wearing a knit cap, hands in his pockets; but it's 80 degrees out and the sun is shining. Be it right, wrong, politically-incorrect, or even illegal for you to profile... if that put you the least bit on edge, you just (wisely, I think) profiled someone. He may have a completely legit medical reason for being dressed that way and you may be waay off base in your assessment. But maybe not.
You pull up to your neighborhood 7-11 at 11pm for a gallon of milk and see 2 17 year old guys in tanktops, covered in tatoos and piercings, and sporting multicolored mohawks standing on either side of the front doors. They might be waiting on "Mom" to come pick them up after a movie. But something just doesn't quite "feel right", and you drive a mile further away to the Circle K. You just profiled.
My point is this. If you are at all aware, you notice things that stand out as being "out of the ordinary", because they are, and you alter your actions accordingly.
Now, back to the original post(er). Granted, your AR15 is legal; as is you're keeping it in a rifle rack in the back window of your pickup. Granted further that any knowledgeable person will reasonably believe that it is just another semi-automatic rifle, and that neither it's shape or color makes it any more dangerous than any other semiauto rifle. You may look like Wally Cleaver and have graduated at the absolute top of your class. But THAT rifle, displayed openly in your back window, IS going to draw attention; everytime, everywhere, and from everyone, without fail.... from gun-loving folks, gun-fearing folks, and the Police. That Cop (if you've reported your interaction accurately) could have handled the situation better. But by displaying it as you did, you virtually "begged" to have someone comment on it. Period.
In my State (Wi), believe it or not, it is illegal to carry concealed. In my little town, (Wausau), it IS legal for me to WALK down the street with my 1911 in a holster so that it is in plain view. However, doing so "may" subject me to arrest if another citizen calls the Police about it, due to Public Nuisance and other laws. It is a big gray area, and not something I care to test. The point is that it's going to draw tons of unwanted attention to me.
Your back window display also screams out thieves, "Follow me home. Bet'cha I have other weapons there for you to steal." Further, you will know nothing about them, and you will have at least somewhat educated them about you. Bad idea.
So take the whole thing as an educational experience. Officer Happy may have been a rude prick. But his question regarding "Have you ever had a gun in your face?" is worth thinking about. The 'gun in your face' might not be in the hand of a Cop, but some lowlife that wants yours, and isn't going to let you have the time to use yours.
You "profile" people all the time. You are also being "profiled" yourself.