And he did see a weapon in the car.
In some states, you must inform an officer if you have a loaded (maybe even unloaded) firearm in the vehicle, whether asked or not.
In other states, you must confirm you have a firearm if the officer asks but you do not have to volunteer the information.
So right there, the officer probably had the right to arrest him, since one or the other of those probably applies (cant say for sure tho).
Merely asking if you have any weapons in the vehicle isn't a violation of a person's rights. Now, if he had asked to search the vehicle based on those bumper stickers that could be an infringement.
Every time I've ever been pulled over by the police I've always been asked this question. Sometimes I do, sometimes I don't, sometimes I've forgotten and have to amend my statement. The last usually ends with another 15 minutes of hassle but for the most part, how you interact with the officer is the most clear indicator of how the officer will interact with you.
This dimwit holding a piece between his legs shows me he deserves to be arrested. For being stupid. If you've already told the officer "No" and have no intention of amending your statement it's best to leave that weapon wherever you had it. If it was always in between his legs he could have found numerous ways to better conceal it after the officer went back to his cruiser. So as it is he was dumb.