Nobody seems to have picked up on the "external" spam filter part of the response.
My guess is that Y! uses some somewhat dodgy external providers of spam/content filtering. Historically, external providers that block inappropriate sites have considered such lists to be their proprietary information, but when those lists have been revealed, they tend to have a pretty heavy political agenda.
I'd be a little surprised if Yahoo uses third parties for spam filtering. The Twitter thing is no mystery, though; Twitter routinely hides controversial hashtags.
My guess is that Y! uses some somewhat dodgy external providers of spam/content filtering. Historically, external providers that block inappropriate sites have considered such lists to be their proprietary information, but when those lists have been revealed, they tend to have a pretty heavy political agenda.
Peacefire has some reports that detail this:
http://www.peacefire.org/
(So, I think the reason for the Y! block was likely an unaccountable third-party, but I think the Twitter block is pretty odd.)