Poor spot for the captain to find himself in; I don't envy him.
Just guessing, but it's easy to imagine that the local cops treated the incident with less interest than at any first-world airport. "Did you start the fires?" No. "Did you see anyone start the fires?" No. Next passenger. They obviously didn't deprive the perp of his/her incendiary material.
I suppose after the next in-flight fire I'd consider diverting to the nearest "suitable airport" being one stocked with a large contingent of TSA-like folks with nothing better to do than lock up people who like to mess with aircraft. Sure would be (alas much too) hard to convince ops to go along with that.