On the Trademark Blog Martin Schwimmer notes an antitrust case that has a nuanced view of relevant markets for antitrust based upon trademarks (Exclusive Licensed Products May Constitute Separate Market For Antitrust Analysis):

The defendants had argued trademarks only indicate the source or sponsorship of goods or services and say nothing about which other products consumers might see as substitutes.

While that is usually true, the judge said, here the NFL team logos can be viewed as the actual product. While some people might buy a T-shirt or hat with a logo just because they need a T-shirt or hat, most people who buy logo-bearing merchandise are fans who would not see other kinds of apparel as a reasonable substitute, the judge said.