I can't deny that DDoS attacks skirt the edge of anarchy, given the critical importance of the network-reliant, continuous connectivity required for supporting the world's business and social communication, security protocols and various "media" channels.
So when, if ever, is it acceptable? My gut says..
Maybe when the unfair advantage of an overwhelming media messaging machine, such as that funded by the Kochs (Dixie, Vanity Fair, Quilted Northern, etc..), is leveraged against the overall health of the vast majority of a society's members. Then, fighting back in ways that may be legally dubious seems more like an attempt at leveling the playing field.
Objective observers might ask why the legality of the near-monopoly of media outlets, being exercised by corporations and the wealthy upon us, is not the issue/question. (see '1984')
I might feel a little differently, and a bit more generous/flexible, if the US DOJ's actions and the Supreme Court's patterns of decisions revealed anything other than priorities of self-preservation and partisan politics, respectively. I expect there to be a "Legality Marketplace" being setup sometime very soon, now.
The cumulative actions of many small agents, ridding an organism of an internal component that is secreting poison, strikes me as the morally, if not legally, correct (..pro-life?..) action. Even when undertaken at the risk of imprisonment over a primarily symbolic gesture. Of course, who can say what symbolism ..Tunisia/self-immolation, anyone?.. will have a larger meaning and significance.
I'm sure the Tea Party minions have their version of the above justification, with somewhat different targets. But my confidence that history will view them as less than astute analysts of ambient political realities is extremely high. That is, if future generations are even allowed to study history.
Orwell/Blair would have found these to be interesting times indeed.