Just one thing: that's a "corrido" or to be more accurate, a "narco-corrido", not a mariachi song...biggest differences are the subjects (corridos are, like you said, about death, crime, drug-dealing, vendettas, etc., mariachi songs are usually about lost love, good times and friendship), corrido bands often use electric instruments (bass, guitars) and drums while mariachi music is mostly acoustic with guitars, trumpets and violins. I could go on and on but you're right in your last sentence: asking why Mexicans would come out to Mariachi is also akin to asking what's up with them eating tortillas.
This is mariachi music:
YouTube - "SON DE LA NEGRA" MARIACHI NUEVO TECALITLAN