The causes of Garner's death were (a.) his resisting the police, (b.) his poor health, (c.) the failure of the police to ease up and give him some breathing room when he said he couldn't breathe, and (d.) the fact that the state has many laws criminalizing free, non-rights violating activities. Take any of those away and he wouldn't have died. If libertarians emphasize (d.), then good for them. If they deny that (a.)-(c.) were causes, as well, they shouldn't.
But those who react to the libertarian emphasis on (d.) by denying that (d.) was a cause (for example, some of the commenters in this thread) are also off the mark. Suppose the law Garner had violated had been a law criminalizing the reading and dissemination of any literature advocating limited government or the American Constitution, written by or about the Founding Fathers, or not approved by the state's Department of the Censor. You wouldn't want to say, "Garner did not die because he violated the reading laws. He died because (a.)-(c.)." The fact that the law Garner actually violated was less egregiously statist than these reading laws doesn't change the fact that (d.) was a cause. There is a debate to be had about how much we should emphasize (d.). But clearly (d.) was one of the causes.
What about "emphasis"? There's nothing wrong with it. It is a political matter. If you want to bring to our attention that the state has too many illiberal laws on the books, then emphasize (d.). You won't be violating any rules about describing causes.
As an aside, consider the case of the murder of Hannah Graham. The young lady decided to take a long walk downtown at 1:30 am all alone with no means of self-defense. That poor decision is one of the reasons she died. In this it is similar to Garner's poor decisions to neglect his health and to resist the police. We should like to emphasize the other cause: her murderer's actions. But there is no sense in denying that Graham's poor decision was also a cause. I have heard some say that it is outrageous to state, "She shouldn't have taken that walk." But they won't tell their daughters it's okay to take such a walk. Punish the murderer, but don't get so upset that you lose your grip on common sense causal reasoning.
The lesson is that you shouldn't let your political views or moral outrage distort your diagnosis of cause.