The Digital Economy Act he is complaining about in that article - which, like SOPA, included provisions for national censorship in the name of copyright - passed by a large majority. Most MPs were not present for the debate, but simply voted according to their party line.
I can't find much on Berners-Lee's stance on SOPA. Do you have any information as to why he wouldn't sign this letter?
EDIT: Just realized he is British (thanks to the other reply). Do you believe his silence (if any, again I can't find much on his stance) relates to his citizenship?
There's no citizenship requirement for petitioning Congress, the petition doesn't identify them as exclusively US citizens, this is an issue with international repercussions, and they're prominent figures that even the dolts in the House may have heard of. While nationality is likely the cause, they are no less conspicuously absent.
