Because it has a sewer hose connection, I suppose the intent is for you to use a wye connector in your sewer hose so that it all drains to the park sewer. Which probably works just fine if you have hookups. More and more the manufacturers are designing FHU only capable rigs. Especially the higher end units. Not surprising really when I read here on RV.net of so many people that never boondock. I couldn't live with that, but others are perfectly happy.
It doesn't drain to your gray tank because you are not expected to use an outside kitchen while traveling. Makes a little more sense when you think about it in those terms.