Discrimination! Pass A Law!
It's a good thing gun owners (unlike certain other groups) don't insist on laws banning discrimination by private businesses. Instead, we take a more polite approach: we tell businesses that don't want our business that we'll go elsewhere. The Smallest Minority has a nice article about politely leaving a note on the subject.
If Idaho had enough businesses to bother with such cards, I would keep them in my wallet. But with the exception of the Edwards Cinemas in Boise, I've never seen any "no concealed weapon" signs on businesses here in Idaho.