Do not take it the wrong way...

I am sure you were not intending to be rude and only asking questions. But just like the officer... things that are said by each party can and will be viewed as "rude"
You did not like how he spoke to you. Speaking down to you... making threats. He did not like some 21 year old kid questioning him and accusing him of being a liar in regards to the policy and trespassing (My Speculation).
To me... he was not rude. You have a 40+ year old cop speaking to a 21 year old kid. Sorry..to us older people you are still a kid.
You also have to also remember that... someone of authority needs to make it known that he is not going to let you walk all over him so his being firm is not intended to be rude. This goes for ANY person in that position like your boss.
But truthfully... neither side was really rude. They just did not like what the other was saying.
I would say he was pretty much prepping you to leave when he approached. He did not just walk up and kick you out or demand your ID, right?
So you said something.... maybe you did or did not tell us about... that set him off and caused him toeject you immediately! Or maybe he was having a bad day.
Were you trespassing?Not if it was just a policy. But now you have been banned so if you return.. armed or not.... you will be trespassing.
I am confident that had you told him "I was unaware of the policy. Would it be OK to just go put it in my car?" He would have said "Sure"
But you said
Me: "Actually I'm not trespassing. There were no signs out front, and nobody has told me to leave."
This became a challenge situationand he is going to win.
And he did.... legally and lawfully.For your "rude" behavior.. The officer paid you backand with interest...
Interest being... You are now forbidden to return while armed.
Edit.. Added while armed