He may feel there exists that statutory authority to arrest under Louisiana law. I am interested in looking at the statute first before I prejudge. The truth is that there are statutory and regulatory limitations on free speech and assembly that the courts have determined to be constitutional. Maybe he has one. The arrests are likely to sort this out,as judges begin to see the suspects and the charges and does a little legal research. The arrest examination is part of the clarifying process in law.
The Supreme Court already ruled on Westboro, they can still protest outside a buffer zone between them and the funeral legally. I believe La. has such a law that implements a buffer zone, and if it's violated the demonstrators are subject to arrest.