The U.S. military has had for some time a policy of inducting non-citizens in times of military need who were also looking for a fast-track to citizenship. Many Latinos no doubt have already availed themselves of the military's open-door policies. I'm sure there are many motivations for "upping", not the least of which would be weapons training not available elsewhere for an aspiring revolutionary member of la Raza on American soil.
Your right, except 99% of the military isnt foreign citizen and these gang members are more than likely born and raised Americans.