Can open source games be published without violating the license? Yes; because open source games have been published, and Steam allows it.
Simutrans is a simulation game that is offered through Steam, and advertises itself as open source. It may very well be the only open source game that has currently been published; There is actually a search filter to look for open source games, but games are tagged by the community, so it is not an exclusive list. Warsow is another such game that is currently in Steam Greenlight, but has been published through other means.
Pikalek has brought up one very big example of open source games being available through Steam; many of the older Doom, Quake and Wolfenstein games are available through the Steam store, despite using open source licenses.
While I can not personally comment on why we do not see many open source games, on Steam, I found a comment on Reddit that offers some sound reasoning as to why we do not see many, and other complications that may arise:
Because to put something on Steam there have to be some person who may represent project in relations with Valve and many open source project don't have such people.
Technically license of open source games allow anybody to put their compiled code and assets on Steam even to sell them, but there is open question about trademarks. E.g even if trademark not registered ownership of project name technically belongs it's original creators so likely Valve not going to accept project to Steam if it's use same name, but not submitted by actual developers.
Other question that many open source games licensed under GPL and it's make it harder to use Steam integration as Steamworks SDK is proprietary and can't be linked with GPL'd game executable. In same time Valve don't provide easy way to use Steam-only features (as far as I aware WebAPI it's not exactly what can be used in game).
- SxxxX (posted 16/2/2015 and quoted as written) @ /r/Linux_Gaming : Why are many (free and) open source games not on Steam?
