If a bunch of script kiddies (albeit in considerable number) can take down MasterCard, then it's pretty much MasterCard's fault alone. It's not like they didn't have the money or opportunity.
LOIC is really, really basic, and anon has never really gathered any respectable amount of bandwidth. Most small botnets put them to shame.
What I suspect, however, is that actual botherders are using these raids as cover. They can still DDoS the target, but 4chan takes the blame. The recent addition of "hivemind" functionality to LOIC, which slaves the user's computer to an irc feed that controls targeting and firing, seems like the perfect opportunity for a botherder to set their botnet to take orders from same.
And attacking a website is never justified. It is wrong regardless of how much effort the owner puts into securing it. Likewise, it's exactly the same crime to steal a car that has an alarm as one that doesn't.
As to your specific point, leaving my car unlocked with the windows down overnight on a dark street in Detroit doesn't make the inevitable thieves justified, but it does make me retarded, and completely undeserving of sympathy.
edit: The only difference is in how fast you manage to recover.