Having said that, statements like "We figured that there’s got to be something in our Constitution that would guarantee our shopkeepers the right to speak openly about political issues…" are moronic. Anyone, even political parties, have the right to protect their intellectual property.
If you do not protect your copy righted material it becomes public domain and you loose all rights to control its use. It then can be used and abused by anyone. It is similar to squatters rights.
IP law folks feel free to chime in (IANAL), but it seems there were two ways to respond in order to assert control of the trademark. Option 1 - send a cease and desist letter. Option 2 - acknowledge the usage as licensed. If this went to litigation it seems like all of the anti-GOP designs would have been protected speech and all of the pro-GOP designs("I'm raising my children right!") would have been removed.
A strict interpretation of the constitution would assert that protecting freedom of the press is more important than "intellectual property", considering the 1st amendment specifically asserts "Congress shall make no law [...] abridging the freedom of speech, or of the press". Article 1 section 8 may empower Congress to establish copyright, but the power granted must stop short of the 1st amendment.