They're viewed a threat because quite a few of the traditional street gangs of African American, Mexican, Dominican, Puerto Rican, Caribbean (Jamaican or Haitian), Chinese and Vietnamese backgrounds always took a "money first, mayhem later" approach to the point that they definitely made major inroads into organized crime. Of course they whack each other left and right, but they're mostly focused on business.

Now you got those "new" street gangs...like Southeast Asians (Cambodians and Hmong for instance) or Somalis that aren't nearly as successful as far as graduating to OC is concerned, but are definitely a major public nuisance because of the way they love to create all out mayhem and terror. The thing is though, that those gangs lack numbers compared to the traditional ones. With the Central American gangs like MS-13 the thing is that they also take a similar approach in seemingly not caring all that much about money making and "professionalism" and prefer to bring the terror, but that they also are HUGE in numbers. That's definitely a cause for concern to about everyone.