Ideological differences are no excuse for rudeness.
Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]
“Miss Manners,” syndicated column (1978-10-08)
(Source)
On interactions between the general public and picketers, though she has used the phrase on other occasions.
Reprinted in Miss Manners’ Guide to Excruciatingly Correct Behavior, Part 3 "Basic Civilization," "Common Courtesy for All Ages" (1983).