I do not think that the employee's response was proper. It was an unprofessional, emotional reaction.

That said, I note that Mr. Kidd initiated the exchange by volunteering, unprompted,  a politically charged statement to a complete stranger in an inappropriate venue, and he should not have been so distraught when the reaction was negative. That's par for the course when doing that sort of thing.

Also, nowhere in Mr. Kidd's telling of this story did I see where anyone was trying suppress his speech, so I'm not sure what the relevance of the Voltaire quote is.