Constitutional right are rights that are protected from interference by the government. That's basic constitutional law. Another citizen can take action against you because they don't like the content of your speech, for example. They can kick you out of their home, place of business, or whatever. It's the government that can't do that.
I personally think it is stupid for businesses to bad weapons from private vehicles if they are locked in the trunk. But, I also think its their property, and they have the right to set their own rules. If you don't like them, you are free to find another job or patronize a different business.
Know what? You are absolutely correct. But you knew that. I stand corrected. But that takes us right back to the public schools and my right to have a gun in my car on school property. I teach 7 hours a day and stay on campus to grade (on my own time) for 2-3 hours more each day. I teach in the inner city. I should be able to protect myself on those streets when I roll out of the parking lot....