Lets make this clear - SOPA, even the revised version, calls for Internet companies to build a technical system that can be used to deny people the ability to view certain foreign websites.

Presently, you offer a narrow range of sites that might be blocked via this mechanism.

However, once this infrastructure is in place, other groups, besides copyright interests, will want to use this infrastructure to ban other types of content. This won't strictly happen at the federal level, state governments will also get into the act. It will be easy to add to the list of banned sites once the infrastructure is in place.

Initially, the categories of things that are banned will probably be quite narrow. Very little content is actually illegal in the US, and so there are very few categories of foreign sites that could be Constitutionally added to the list. But eventually, everything that can be added, will be added, and some things that shouldn't be added, will also be added, and there will be controversies about that.

Then, one day, there will come a crisis.

Perhaps, classified information will find its way onto a website outside the US, as occurred with Wikileaks.

The government will find it attractive to use the infrastructure that SOPA created to ban access to this website in order to deny Americans access to this information, which may be politically relevant and available to citizens of other countries.

In the Wikileaks case, for example, the Department of Defense warned employees not to access classified information on Wikileaks, asserting that it was still classified even though it had been widely disseminated. Joe Lieberman called for payment processors to stop processing payments for the charitable interest that funded the Wikileaks site. It is not hard to imagine these interests, in a similar situation, calling for the site to be blocked if an infrastructure for blocking access to foreign sites was readily available.

Doing so would have created a situation where Americans were prohibited via a technical mechanism from reading information relevant to their government which is readily available to people who live in other countries.

In a country with a strong commitment to the freedom of speech, and open political discourse, this sort of government paternalism with regard to politically relevant information is unacceptable. Perhaps such an action might even be found unconstitutional after a lengthy court process, but a temporary ban on access to information, even if later considered illegal, might nevertheless be effective at furthering the interests of those who wish to keep the American people in the dark for a time, and would be consequence free for those who established it.

That is why SOPA is not acceptable, and will never be acceptable regardless of what changes you make to it. SOPA creates an infrastructure for censorship, and that infrastructure will eventually be used, regardless of whether or not that use is ultimately considered legitimate in hindsight. The only way to prevent illegitimate use of the sort of censorship infrastructure that SOPA would create, is to avoid creating it in the first place.