We can make this argument by making two main points.
First, it is very problematic for the government to ban speech that can be seen as political. Westboro Baptist Church chooses to use horrible language in making its point, but its point can still be seen as political. It is arguing that America’s moral fiber is weakening. This can clearly be seen as a political point of view.
Second, the manner in which the church pickets the funerals is not disruptive enough (one can argue) to warrant banning the speech. Speech can be banned if it is done in an inappropriate time, place, or manner. But we can argue that the Westboro protests do not actually disrupt the funeral.