It's a matter of semantics.

Copyright does not protect names, titles, slogans or short phrases. However, slogans or phrases can be legally protected as trademarks.

http://www.copyright.gov/help/faq/faq-protect.html

How do I copyright a name, title, slogan or logo?
Copyright does not protect names, titles, slogans, or short phrases. In some cases, these things may be protected as trademarks. Contact the U.S. Patent & Trademark Office, However, copyright protection may be available for logo artwork that contains sufficient authorship. In some circumstances, an artistic logo may also be protected as a trademark.

A trademark, on the other hand, is a word, phrase, symbol or design, or a combination thereof, that identifies and distinguishes the source of the goods of one party from those of others. These can be protected using the US Patent & Trademark Office. http://www.uspto.gov/faq/trademarks.jsp

I think the only question, and I'm not a lawyer, is can Chrysler sue because it has a "pending application" for a trademark and not an actual trademark on the slogan? If the application is approved - it's a moot point and Pure Detroit couldn't make the shirts sans Chrysler approval. In between approval or rejection- that's a question for the JD's among us.