Yesterday on Twitter, somebody asked about how DNSBLs are restricted by the First Amendment to the United States Constitution. Apparently there's some advocacy group somewhere who is upset that they're listed on a blacklist, and they seem to be investigating potential opportunities for legal recourse.
For reference, allow me to include the entire text of the amendment here:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
Okay, now that you've read that, let me ask you this: Which anti-spam blacklists are run by Congress? The first amendment restricts GOVERNMENT action, not PRIVATE action.
(Sure: Other laws certainly restrict various private actions, but that's not the point, is it?)