If Sebastian only had funds stuck in Weexchange(or if he is mostly interested in what happened to the rest(5870? BTC) of the  ~6256?(386/0,0617) BTC in weexchange) and not in Ukyo loan, with Weexchange beeing registered(?) in Australia the logical would be to send the letter to the Weexchange's adress in Australia and not directly to Ukyo even if Ukyo that has handled most of Weexchanges business is in the Usa and it's Graet that is in Australia.
Or maby it's just because it's simpler to handle a process within the same country as he's using an Australian lawyer.