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).