I could go into a long discussion of Japanese culture and the relationship of Japanese society with the government, but... Really, you can't expect any large organisation to act rationally. All such organisations are made up of many, many people all of which have their own agenda. The action that is expressed at any given moment depends entirely on the ebb and flow of internal power struggles. You can't really predict it or reason about it in the same way you might for a single person. It could be anything.
I'm not OP, but I would be very surprised if the attack was the root cause for strengthening their spy agencies. In the United States we have seen time and again that laws for a stricter surveillance are passed with the pretext of anti-terrorism or anti-child pornography; Japan could very well be using such a case to justify their surveillance needs.