Unfortunately, it's not. It's just a great side effect of the right to a trial by jury.
In the American legal system, the modification of laws is left to the legislature. A jury can express its displeasure with a law by nullifying, but this has no legal import, since a different jury could easily convict. Thus, the primary method of expressing displeasure in a law...is to express displeasure in a law (by calling your local legislator).