As ever, the blame is apportioned across lots of people, none of whom can be held aloft singularly as the culprit, and none of whom can be wholesalely changed to solve the issues. If Sony had better engineers, this wouldn't happen. If the engineers had better management, this wouldn't happen. If the server software companies had better testers, these flaws wouldn't get through, and if the testers had better management and tools, they could do a better job. And, of course, if the hackers had any principles, we wouldn't have to worry about security flaws in the first place! And if they had better parents. And if the parents had a better, more supportive society...
If it were possible to isolate one part of the chain and make it perfect, we could solve all the problems of hacking. But that is not possible, and there will always be crime and victims.
It's the same as console piracy. Each generation the console companies implement security measures to prevent piracy, and each generation some new exploit they never considered is found. The next iteration of hardware fixes those known faults, only for some new vulnerability to be found. Same with internet services - you can only protect against known issues and expected issues. Trying to defend against and every unknown possible attack vector is impractical.
The question here is whether the hacks are because Sony at large was complacent/incompetent, or if they were just a victims of the ordinary run of cat-and-mouse hacking. If, like the PS3 encryption fault, Sony were sitting on weak servers that they knew were weak, then they didn't do as much as they realistically could and need to shoulder a lot of the blame. But if their security was 'as good as every one else' and they were led to believe that they weren't open to hacks, then it's not particularly their fault if they couldn't preempt security measures that would have prevented this. At the moment there's lots of noise saying Sony were incompetent, but there's no hard evidence yet, but most people seem to prefer to judge long before the trial is ever held.