I'm not surprised, especially in the case of Latino gangs: if a symbol pervades a culture, it's likely to be seen as an identifying mark of that culture, regardless of the meaning that others place on it.
And I think that there's a long association of the cross with violence, anyhow. According to popular myth, it first gained widespread acceptance because Constantine used it as his battle standard (and before anyone says anything, I know that he actually used the Chi-Rho, not the modern cross, but I'm speaking of the myth here) - Crusaders painted their shields with it. The French version of my own country's national anthem has a line that translates to "as our arms have carried the sword, so have they carried the cross."
You can say that this association of the cross with violence is a corruption of the Christian message, but again: plenty of nasty, violent people were using the cross as their symbol long before the gangbangers came on the scene.