Hispanic isn't actually a race - it's an ethnicity. There are white Hispanics,
black Hispanics and Hispanics that don't identify as either black or white. Therefore, it is illogical to take issue with that description on the basis that it is inaccurate because it isn't necessarily inaccurate.
It is logical, however, to question why people emphasize the white "side" of Zimmerman in this story when, if he were perhaps involved in another issue, the same people would emphasize his "Hispanic" side. For instance, if Zimmerman was in the news for being stopped in Arizona by police who questioned his immigration status, his "Hispanic-ness" would probably be emphasized over his "whiteness" whereas now his "whiteness" is being emphasized over his "Hispanic-ness".
I suppose some people will argue that it's just a simple case of hypocrisy and baiting where the media uses whatever identifier makes the better the story. I, however, would argue that it is more complicated than that. Because race is a social construct, how people identify each other racially can often change in certain contexts - not out of a conscious effort to be deceitful - but instead out of their own perceptions of how race changes in certain contexts. For example, consider how some black people can "pass" for white. In certain contexts, they are considered white and in other contexts, they are considered black. This happens with a lot of people.