Where are you getting your information that says this rent a cop or anyone for that matter has the right to prohibit a citizen from displaying a sign on public property?
And just WHAT does your claim the rent a cop was not “very far out of line” mean? You either are or are not out of line.
Since it’s public property, and a public occasion, I would naturally expect and understand that the civic authorities and their agents have the right to set and enforce standards of behavior in that time and place.
“not very far out of line” is a statement that self-labels the speaker as a "moderate" and who, therefore, has a duty to provide pontificating balance in every decision-making situation.
The kind of person every defense lawyer strives to get on his defendant's jury...(Think Lyle and Erik Menendez's first two juries....)