2010 Census shows "United States (R)" - registered trademark. WHY?

In my Census packet (and probably everyone else's) there is a half-sheet notice included - from the United States Department of Commerce. At the very bottom of this paper, in the lower right corner, it says "United States (R) " i.e. the R with a circle around it, meaning it's a registered trademark. Why is United States a commercial entity now, and since when?