That's funny. You don't "believe". Let's see what the Steam ToS have to say:
When you upload your content to Steam to make it available to other users and/or to Valve, you grant Valve and its affiliates the worldwide, non-exclusive right to use, reproduce, modify, create derivative works from, distribute, transmit, transcode, translate, broadcast, and otherwise communicate, and publicly display and publicly perform, your User Generated Content, and derivative works of your User Generated Content, for the purpose of the operation, distribution, incorporation as part of and promotion of the Steam service, Steam games or other Steam offerings, including Subscriptions. This license is granted to Valve as the content is uploaded on Steam for the entire duration of the intellectual property rights.
And that's from the Subscriber Agreement, i.e. for what you post there as a user. Never mind if you publish a game with them.