I am just playing Nathan based of what he knows...1) Harbor master is old and no heirs wanting to step up, so it would be bad business for him to take to burning ships. 2) The dispute, as he doesn't know what it is, could be a matter of business and as the Merchant's Friend is his god, he would favor the business side first. 3) Nathan isn't comfortable around nobles to begin with, so his hackles get raised by them doing what they do best, seek to control things. Since he was invited there as a guest, he sees it as his place to make sure she remains in control, especially if there might be donations and offers to the Church coming his way..