Not defending SOPA -- I don't have a clue about its intricacies -- but when a bunch of companies that don't sell software are against a bill that purports to try to do something about software piracy, I don't really see what the news is. In fact, these companies are all basically ad networks...
That doesn't mean you should abandon all reason and blindly trust them. Actually, you should be wary and watch your back - if their interests change, as you say, a company won't think twice and back-stab you without batting an eye.
There's a reason that there's no law that holds telephone companies responsible for law-breaking which is organized over their wires / cell towers. It's ludicrous.
The phone companies have all sorts of legal duties to law enforcement. If a court orders it, they have to assist with taps, traces, geolocation, call records, etc.
As you say, piracy is already illegal. And we know there's a ton of casual piracy that goes on--see:
http://arstechnica.com/tech-policy/news/2011/11/its-official...
So the question is: what mechanisms of enforcement can dampen casual piracy and yet be acceptable to most tech companies?
That's not to say this is a good bill. It's not. But if it's in the interests of companies with significant IP to protect to support it, companies with significant IP to protect like Microsoft, Adobe, and Apple can be expected to support it.
Their support is really that simple.