Because if SOPA passes, any complaint of copyright infringement (and yes, I said "complaint", not "violation", much less "proven violation") means your site gets shut down, no questions asked, until you feed enough money into the legal system to maybe be allowed to speak again some day.
Can anyone think of any particular niche genre of adventure games fan sites that might be easy prey for such a witch hunt? Hmmm? Yes, the OSR will be SOPA's bitch-boy, no free cigarettes or lube recquired.
As per Wikipedia:
"Call your elected officials.
Tell them you are their constituent, and you oppose SOPA and PIPA.
SOPA and PIPA put the burden on website owners to police user-contributed material and call for the unnecessary blocking of entire sites. Small sites won't have sufficient resources to defend themselves. Big media companies may seek to cut off funding sources for their foreign competitors, even if copyright isn't being infringed. Foreign sites will be blacklisted, which means they won't show up in major search engines. SOPA and PIPA build a framework for future restrictions and suppression.
In a world in which politicians regulate the Internet based on the influence of big money, Wikipedia — and sites like it — cannot survive.
Congress says it's trying to protect the rights of copyright owners, but the "cure" that SOPA and PIPA represent is worse than the disease. SOPA and PIPA are not the answer: they will fatally damage the free and open Internet."