Not only is he running a media company (which ostensibly should be in favor of SOPA), but also he's the same generation as the lawyers, lobbyists, and legislators supporting SOPA.
Most other creators are just churning out low rate disposable content. Their business models are struggling under the legal alternatives the internet has come to offer, just look at what happened to newspapers.
Now that distribution is solved, competition has sky-rocketed and they're fighting a market that wants to show them just how little their product is worth these days. Steam may be the internet distribution posterboy, but think of any other type of media producer who looks at steam. There are rock bottom prices on everything that isn't a AAA title released in the last 6 months.
They're stuck fighting the new marketplace realities, they can't afford to see piracy in the periphery (like O'Reilly can), so they do the one thing they know how to: give cash to congress to make that problem go away.
EDIT: Typo.