I think this is the crux of the matter, rather than the 30%.
As much as I enjoy steam and don't care in the slightest that they charge a 30% fee for all they offer, I can't see any general benefit for any of the game companies to be able to dictate how game makers price their goods on other platforms or for direct sales.
If Valve is making access contingent on competitor pricing, I could understand it being seen as anti-competitive behavior. It should be banned across all of the marketplaces if it somehow doesn't fall afoul of some consumer protection law already.
edit:
HideousKojima says below that the pricing only applies if they're selling steam keys. If that's the case, I can't see how this has merit. If you're selling Valve's support of your game to users, pay them what they ask. If you're selling elsewhere and Valve is uninvolved, then there would be an issue.