The practical reason I can see is that: 1) a large portion of Wikipedians from Italy are worried about the consequences of this law for Italian Wikipedians and wish to stage a protest; and 2) editors from Italy constitute an overwhelming majority of editors on the Italian-language Wikipedia, and are therefore able to stage a protest there, but not elsewhere (since they don't constitute an overwhelming enough majority of the other languages' Wikipedias to get consensus agreement for a blackout).
So far it seems the Foundation, which ultimately owns the servers and could intervene if it deemed the move contrary to the organization's principles, is standing on the sidelines. In part, that might be because the consensus is so strong; it wouldn't be overruling a handful of rogue admins, but a large portion of the Italian Wikipedia's editors (straw-poll here: http://it.wikipedia.org/wiki/Wikipedia:Bar/Discussioni/Comma...), which it prefers not to do unless absolutely necessary. A number of people in the organization, speaking in their private capacity, appear to also be fairly sympathetic to the protest, and not inclined to overturn it in a heavy-handed manner (perhaps there will be some informal negotiation to bring the site back online without condemning or officially "overturning" the protest).
So far it seems the Foundation, which ultimately owns the servers and could intervene if it deemed the move contrary to the organization's principles, is standing on the sidelines. In part, that might be because the consensus is so strong; it wouldn't be overruling a handful of rogue admins, but a large portion of the Italian Wikipedia's editors (straw-poll here: http://it.wikipedia.org/wiki/Wikipedia:Bar/Discussioni/Comma...), which it prefers not to do unless absolutely necessary. A number of people in the organization, speaking in their private capacity, appear to also be fairly sympathetic to the protest, and not inclined to overturn it in a heavy-handed manner (perhaps there will be some informal negotiation to bring the site back online without condemning or officially "overturning" the protest).