I agree with your sentiment, there is no way that the general population of taxpayers should be subsidizing education for people with the sole rational being "they're happier" or "they have a deeper understanding of the world". That's great for them, but don't spend my money on it.
However this data doesn't really mean much. The table shows how many poeple have thsoe jobs RIGHT NOW. Will they have those jobs forever? Likely not. If you have a phD and spend a year in a crap job looking for something better, I view that as better than just doing nothing.
In fact those numbers are just people with more than a bachelors degree. Some number of them are currently graduate students, I know plenty of graduate students who are bar tenders at the same time. This data doesn't differentiate.
It also doesn't tell you anything about how school was paid for. If you have a phD that someone paid for from a private school, and do nothing for with your life, that doesn't matter to me at all. Only when people start getting subsidized by the taxpayers does it matter.
And hey, if we're going to bitch about the cost of education to the taxpayer, I'd extend it even to phDs with jobs that are paid for by the government (professors doing research at both public and private universities funded by taxpayer grants). A lot of research is freaking useless, and it costs a whole heck of a lot more than the subsidy on loans for students.