Nope, no design flaw, I think some people just want to direct the flow of smoke. My caution is that it wasn't designed with vents and the airflow WILL change if you add a vent and change as we know isn't always good.
Vents are not required and I only have a vent cap on mine to keep the wind from blowing in as it's so windy in Oklahoma.
I leave two outside all year and all they have is the cover.
Oh, see it's your first post. Welcome!