Yeah I definitely agree that there's no winning in online arguments.
But I'd have to disagree with the visitor/potential customer always being right.

I think with customers as an entity in general and on the whole it's definitely a good idea to treat it as a "customer is always right" environment.
But down to the individual level? Treating every individual customer as if they are always right? It's not necessarily a requirement. And in fact could become harmful to the business on the whole.

I think every business is entitled to have a "We Reserve the Right to Refuse Service to Anyone" sign and enforce it on occasion.