Your working off an outdated strategy, Paradox changed that mechanic quite some time ago.
The reasoning is entirely mechanical tbh, though it doesn't make that much historical sense In short, if a nation controls both sides of a straight, they can cross. It also depends upon who was the aggressor, that adds another layer of complexity.
Also,the AI currently doesn't obey forts zone of control, but that is hopefully changing in the next patch.
Sorry Ara, you go shafted by game mechanics. Though the Turk is indeed a fearsome opponent in EU4.