Oh contraire. It can be.
http://blogs.findlaw.com/blotter/2016/10/gang-affiliation-charges-is-gang-affiliation-a-crime.html
https://www.legalmatch.com/law-library/article/gang-affiliation-laws.html
Although it is not unlawful to participate in a gang, many states have enacted laws targeting gang affiliation to prohibit people from joining.
The definition of a gang varies, especially when it comes to youth gangs. In general, a gang is an organization, group, or association of at least three people.
The gang has:
A common name
Identifying symbol(s)
Identifying sign(s)
Members who actively participate in crimes
What Are the Federal Laws against Gang Affiliation?
The federal government currently does no have any particular laws against gang affiliation.
However, it does prosecute known gang members under the Racketeer Influence and Corrupt Organization (RICO) statute.
Racketeering is defined as operating an unlawful business. This sort of crime is typically associated with the mafia and organized crime.