That’s just it, businesses do not serve everyone. Your lefty local rag is under no obligation to print conservative letters to the editor or even sell ad space to conservatives.
And it will never go both ways. A business that refused to do business with Christians will never face a problem
The posting of a letter to the editor is not a business transaction - and I beg to differ with you about the ad space.
That being said - as far as a normal retail customer:
When business owners hang up open signs, whether literally or figuratively, they have a responsibility to treat all customers equally under the law.
State and federal civil rights laws prohibit discrimination in places of public accommodation for reasons of race, color, national origin/ancestry, sex/gender, religion/creed and disability (physical and mental).