It does make sense, when you think about it. The point of the census isn't to count citizens, it's to count residents. That's why it includes immigrants, including undocumented persons. You're still a citizen, but you're not going to be using public facilities and utilities, so they don't need to figure you into that equation.
If it makes you feel better, remember that you can still vote by absentee ballot. That's where you count, because you are a citizen.