Well Scheldt shipping would have been relevant to the Entente during this battle:
Siege_of_Antwerp_(1914).
So warship, military traffic, banned on Sep 4, 1914, would have hampered the Entente in that fight, which continued until the Entente retreat and loss of Antwerp on October 10, 1914.
I guess during the battle the Entente didn't feel it would work out in their favor to force an ultimatum on the Dutch, compromise their neutrality, or force them to choose sides.
After October 10 and the retreat, the Germans occupied all Belgian ground on the other side of the Scheldt, so the Scheldt was completely irrelevant to the Entente.
So legal niceties aside, those were the strategy-diplomatic factors at play.
Since at raharris1973 pointed out that's about a month before the Turkish/German attack on Russia I'm not sure whether that is a potential casus belli for the allied powers?
It could have been. But I guess then it becomes a question for the Entente of when does it make sense for them to make an ultimatum out of the issue, and one they are truly ready to back-up.
And of course for the Turks it becomes a question of: If we are not actively trying to ensure Entente defeat, how long should we persist in antagonizing them, especially if and when the Entente appear to be the likely winners.