What you say is true in theory. Each and every thing that you buy is produced by someone who has done something worth boycotting.
That's not the point, though. One of the reasons that this happens too much is that they are never penalized for it. Not by the government, not by their customers. They simply get away with it with no losses. And while you can't change that by simply not buying from those that are guilty of something, you should by all means take the chance to use the bad publicity surrounding the GoDaddy's move to support SOPA to send them a message that their customers this. And these customers happened to be exactly the people who SOPA will hurt.
It was a small thing and a drop in the ocean, but at least to some point they were aware that their SOPA support hurt them their bottom line, even by a bit. That's something. And other registrars know that supporting such legislation would also hurt them.
And if people reacted like this more often, you'd even force corporations to do less questionable things. It's not a boycott of a bad company, it's a boycott against a single bad policy to discourage it. If people boycotted companies "selectively" more often, some would use this as an advantage to gain customers over one of their policies, next over the other, and eventually push their competition to do the same.