If the merchants stay away, the navy won't care about the pirates.

Forgetting for a moment about the analogy, if you identified that an island had a really bad piracy problem, your solution would be to... send all of the merchant ships past it? That's extremely Machiavellian, to say the least.

Perhaps. But publishing warnings and then saying, "Job done!" isn't very appealing either. (https://icc-ccs.org/index.php/piracy-reporting-centre/prone-...)

Hence the part about the royal navy.