The fact that they're also taxpayers is irrelevant.  Like other taxpayers, their taxes go to paying for roads and schools and other such services.  And they use those just like everyone else.  

You're numbers for the employer (i.e. taxpayer) funding are certainly way below what the article states.  Without some sort of citation, I'm more inclined to believe the article when it says "The government agencies that employ them chip in as well — for the two largest retirement divisions, the rate is 20.15 percent of an employee’s pay."

Perhaps you're talking about a different agency than the two largest ones the article references.