The idea may also be that even though you might work for the government, you still will use or benefit from government services. A government employee will only do one specialized job at a time. An officer in the military is not doing remotely the same work as an accident investigator in the Department of Transportation and both those people are not working prosecuting criminals in the department of justice. Depending on how local taxes are funded, there are entire regions of the country that would be cash strapped if they could not tax government employees (For example, in the United States, a vast majority of federal government workers live in Maryland and Virginia, and those states would be out a lot in taxes.
It's the same principle as asking "Why should a childless couple pay the same tax for the school system? They aren't getting anything from it as the couple that has 10 kids!" Just because you aren't benefitted from a government service directly doesn't mean you don't have a benefit (One of those kids in school now is going to be the nurse helping you live comfortably in your retirement home, and he or she is getting that education right now in a public school.).
I'm all for paying as little taxes as possible, but that doesn't betray the understanding that if a large swarth is exempt because of the income earning job they have, stuff that is a benefit to me does not get funding.