I guess you could call something like that "cyber terrorism". Anonymous is spreading their opinion about a political issue by destroying stuff.
This is not changing anything. Do they really believe they are helping?
I can't even tell what they are trying to achieve. Are they trying to impress the masses by showing what they are capable of?
In that case you could compare their "activism" with the defaces script-kiddies do on random websites.
I guess some people who act under the name of Anonymous have good intentions but the vast majority doesn't.
You certainly could call it cyber-terrorism, which is why I vehemently disagree with ineffective measures such as these. They accomplish nothing except to give the government ammunition to use in the fight to expand its scope and power over online activities in the name of fighting some new, vague threat.
Anonymous, you're wasting your goodwill on this bullshit, and in the end, you're almost certainly just hurting all of us. Just stop it.
Terrorism as a word is far too strong here. It rather falls into the area of vandalism, I'd say.
Or are you feeling terror at the thought of a website defacement? The use of "terrorism" is quite inflationary these days, just as "war against.." was before. We have to take care, this can easily lead to more 'anti-terrorism measures', since there is so much more 'terrorism' (because all kinds of 'terrorism' are unquestionably horrible!). (I'm not saying there is no terrorism, but.. let's just be careful how we use the word.)