I would hope no one would neg you for asking a question. He might have been affiliated with the gang in the past, and maybe still is who knows.
My guess would be that he got it because he is proud to be Mexican, and in a lot of places in his home country (USA) Mexican's and people of Mexican heritage are looked at as second class citizens (blatantly or subconsciously).
That can apply to all latinos that have a more "ethnic" look.
Maybe he wanted to show pride of his roots, similar to a 2nd or 3rd generation American of Irish heritage getting a four leaf clover tat. So my guess would be that he got it in his late teens or early twenties when people make haste decisions and just stuck with it.
On topic, I would love for this rematch to be competitive as I am a fan of both fighters. My fandom edges towards JDS because I like stand up technique and because he is just such a cool cat, but on the same token I like Cain's "put you on your behind and bust your butt" style too.
"Let them see that their words can cut you, and you'll never be free of the mockery. If they want to give you a name, take it, make it your own. Then they can't hurt you with it anymore." - Tyrion Lannister