EA have their games on Steam and Origin and have for years, Blizzard could (and will) have their games on Steam and Battle.NET. I've seen nothing to suggested they were unable to put their games on Steam, but rather decided it wasn't in their interest, given Steam's TOS and/or what they could gain from having their own marketplace.

Steam is being sued by Wolfire for antitrust, but there is no outcome in this suit yet. Unless you're referring to a different suit I don't know about.

Even if Valve loses this lawsuit, it doesn't meant they have to allow any specific products from other companies on Steam. It just might mean a reduction in fees, or an inability to sign exclusivity deals (which are common across the industry, weather you like them or not, and I know I don't). The Wolfire antitrust lawsuit is because they sell other companies games on Steam, to the point that they dominate the marketplace, not because they were stopping other companies from selling games on Steam.