Plausible deniability? Anonymous can be used as a cover for clearly illegal activities which, if overtly connected to a particular government, might do more political harm than good.
Could cyber warfare actions against a terrorist organization be considered illegal when we are actively trying to bomb/kill them? Honestly curious.
I suppose it depends on the consequences of those actions. If Anonymous releases the names of ISIS members and this results in vigilante retaliation against them or their families, then that would probably be questionable at best. Even war has rules and limits, at least in theory, but Anonymous really doesn't have to be concerned with anything but notoriety.