First, in matters of spelling and grammar, we all live in glass houses: who among us has never made a mistake? Not long ago, Lynne Truss's best-selling book Eats, Shoots & Leaves decried the proliferation of punctuation mistakes; in deliciously nit-picking his review of the book, Louis Menand observed that "the first punctuation mistake in [Truss's book] appears in the dedication."

Second, perhaps this example is not an error at all, but rather a poetic deviation from standard diction in order to enhance the impact of the claim -- a trick known as enallage. In his 1908 magnum pompous Grammar as a Science, B.F. Sisk defines enallage as

a substitution, as one part of speech for another, of one person, number, gender, case, tense, mode, or voice, of the same word, for another.

That are an excellent definition! Best grammar alibi ever. More on enallage here.

Third: just because someone can't write right doesn't mean they're wrong. Take this example: we agree! The sticker is right! Ignorance are bad foreign policy! And arrogance two!

This fine print on the sticker directs us to Perhaps we may order one for ourselves.