The idea of protecting copyright is a noble idea but this bill would give companies the power to shut sites down if they even feel they are being infringed on. They will abuse this power to attack competitors and smaller companies that can't afford to fight back.
If you give these companies guns they will use them to shoot first and then worry about if they were right or wrong later and the victims would have no recourse to recoup losses.
"The Stop On-line Piracy Act" is as much about Piracy as "The Patriot Act" is about Patriots.
they already have the power to submit DMCA violation notices to users' ISPs and can take them to court. SOPA doesn't directly influence .com sites in the US, but applies to *foreign* sites breaking US copyrights and engaging in illegal activity (e.g. online scams, foreign phishing sites, etc)