It seems odd to me as well. They would be impacted by the legislation because as a domain registrar and DNS server, they'd have to invent and implement the technology to comply with SOPA (e.g., taking down a website).

You might think they have something to gain monetarily, but I'm having a hard time thinking of what that might be. They may take control of revoked/blocked domains, but those domains would have little to no value (since they are blocked) to resell.

The only thing I can think of is just that Bob Parsons or senior management agrees with it on a personal level and used the company name to back it up.

I do know that Go Daddy pretty aggressively defends it's own trademarks and copyright, but even that seems like a stretch to justify supporting it.