You keep referring to "Manned Space Flight". NASA and the JPL are involved in much more than just that. "Manned Space Flight" is the public face of NASA, the part that gets the public to approve of NASA existing in the first place. The other things NASA and JPL do, have a much larger impact on us, but the general population doesn't identify with those programs.
If Congress tells us, "If we pass this bill/expenditure, we'll get a cure for cancer and a 42" flat screen TV for everyone", the public will be saying, "Yes, yes, I want a TV!"
