It was a tricky situation, that much was obvious. Ducarte had to allow the ferry to run; it was a good investment - L'Olius had made sure of that. Yes, there were bridges across the Sil in other spots, but out in the estuary, a ferry was a logical solution.
And, now that the Atwi were land-locked, letting them operate the barge was a solution for everyone. It allowed the natives to have a base where they could access the sea, and it ensured that the connection was managed by a neutral