I wouldn't be so sure. They are spread all over the world and have a rather disjoint corporate structure (as previously noted, Sony Pictures is distinct from the Sony running PSN), so while some branches may be quite secure, others are very possibly not. I didn't hear the technical details of the latest Sony exploit, but somehow I doubt they were that impressive.
Given that a lot of their sites have been broken into with relatively unsophisticated means, such as SQL injection, I think that many Sony sites are certainly in the "low-hanging fruit" category.
Maybe in terms of vulnerability, but certainly not in terms of importance.
Low-hanging fruit simply means "reward" for little effort. The low-hanging fruit may very well be the biggest, sweetest of the bunch.