As far as I know, however, they have been. There are plenty of instances that try and paint Martin as a ne'er do well who liked porn, weed, and was aggressive. The thing is, all of these reasons suck. That says nothing about whether or not he actually did instigate Zimmerman's response, regardless of whether or not he was capable.
But Zimmerman's fucked, because he's the one who killed Martin, and there's not enough conclusive evidence to reassure people that he did it out of self defense.
But this is one of those trials where they're trying to base a decision off a character assessment. Or, in the very least, one that is attempting to manipulate viewers into making a decision based off of the presumed characters of both of the parties involved.