It's going to dramatically depend on department. EPA? Probably a passion, but also not a lot of other places to work for those exact jobs. IRS auditor? Maybe; maybe that's the job they could get, or maybe it's a choice between that and making $$$ at a big-name consultancy. 18F? Almost certainly a choice of patriotic duty, because those people could walk into any big-tech job and make multiple times their salary.