From Reason.com:

The report includes several “visual indicators” to help police determine whether they’re dealing with “extremist and disaffected individuals.” These range from images associated with specific political groups, such as the Oath Keepers and Three Percenters, to a more generic patriotic symbol, the Gadsden flag—a famous Revolutionary War banner featuring a coiled rattlesnake and the slogan “Don’t Tread on Me.” One of the “indicators” is a slightly altered version of apicture popular with fans of the Grateful Dead; the guide does not note this potential source of confusion, describing it only as “common sovereign citizen imagery.”

According to this bulletin a gathering of U.S. Soccer fans could be construed as an extremist rally.