boycotts are discrimination (at least to economists)

Discrimination is a choice for reasons other than "productivity".

The most obvious examples are related to bigotry-- where one decides not to deal with someone on the basis of a hatred for one's race or gender, for example. In such cases, I decide to harm myself by indulging other preferences that I have.

With the recent immigration law in Arizona, people are pointing to racism as a motive for the legislation. I'm sure this is true to some extent. But the "boycott" response is a clear example of discrimination (at least as economists define it). And interestingly, those who are choosing to discriminate are finding it reciprocal, as Arizonans respond (hat tip: Eric Grady).