They are. Nowhere it says that a publisher cannot sell their games outside Steam at a different price. Valve employees then threatened Ubisoft with delisting the game if they did.

In any case, none of the competing stores have dislodged Steam from its role as the most important sales channel for PC games, and Valve employee testimony and corporate records disclosed in the case illustrate the aggressive ways the company has interacted with the developers that rely on it. Emails indicate Valve employees once threatened to delist all editions of Ubisoft’s Rainbow Six Siege “by end of day tomorrow” after they learned the publisher was marketing a separate $15 “starter pack” exclusively on its in-house Uplay store.

From the Bloomberg article: https://archive.ph/YvHxF