The sirens lay their eggs in this dudes hair? And folks are just like "Yum, let's eat!" and not like "Hold up what freaky deaky shit went down between these two and why would we want to put that in our mouths?"
I read it that he was intentionally singing bad. If he sang it well, then the sirens would have never been interrupted. Plus, the lack of anyone reacting to his bad singing makes it feel like they were in one it being intentional.