Eh... Wizardchan is a special place. First of all, it's not Japanese—most of the users there are American or European. Second, forums have a culture and tone. If curious people see posters be angry and mad at society, they will either join in because they share the same values, or go somewhere else. So basically Wizardchan could be considered a subset of NEETs that are angry and sad.

But in the end, I do agree most NEETs aren't happy. I think when we hear "not working or studying" we tend to think "vacation", not depression/loneliness/being ill.