SOPA's main objective is to give the US government an easy, legal way to shut down sites for a variety of reasons.
Piracy is a pretense.
Piracy is a pretense.
We don't have a time machine, so neither of us can actually know the effect. So how are you figuring it out? Legislation by itself has no effect – it's just a piece of paper – the enforcement is what counts. And how a law is enforced is based on its intent (its real intent, which is not necessarily the justification given to the public).
So you're figuring out its "effect" based on the inferred intent, then turning around and saying that the authors' intent is unknowable.
Also, laws aren't always used the way they were originally intended, because laws tend to outlive the people who wrote them. But how, when, whether the law is ever actually used is already very much beside the point. The point is that the government would have the ability to do a certain thing, not that they actually would or wouldn't in any given scenario.