Some people in this thread are way the ■■■■ off base. This is a legitimate thing. It’s not anywhere to be found in the most popular f2p list, which should only be arranged by concurrent player count. Plenty of games up there have way fewer concurrent players than ToS.
If you look at the popular tags for the game, free to play isn’t one of the top tags unless you extend to see the full tags list. That could be the reason why. And it probably has to do with most of the tags being added by people who bought the early release version, so the game never actually got marked as free to play.
Maybe if the devs are notified they can get in touch with Valve to rectify this.