NGCondo is right, you know. It's not "clearly" stated because you have to be looking at the premium page to see it. If you are trying to look at and learn a game, it's not going to occur to you to look at that page, especially someone who doesn't even know that page exists. I had no idea that list existed and I'm the type of person who looks around and tries to find things - I actually
read, even the FAQs, and still didn't know that list existed. It's obvious it's there when you already know it's there.
"Clearly" stated means it should be on the game page. If someone is wanting to look at and learn a new game, they will look at the game page, and it's normal to expect the game page to say if it is premium.
When I wanted to know, I started an invite to every game I know, knowing I'd get a message if it was premium, and marked my own list. But that's kind of finding out the hard way... especially for someone fairly new who might not even know about it.