Because's it's basically in their job description? https://www.nasa.gov/offices/ogc/about/space_act1.html#POLIC...
"...Objectives of Aeronautical and Space Activities. The aeronautical and space activities of the United States shall be conducted so as to contribute materially to one or more of the following objectives:
1) The expansion of human knowledge of the Earth and of phenomena in the atmosphere and space. ..."
Well, if your low-flying and expensive satellite has to pass over the rest of the world anyway, you might as well record what it sees.
Quoth the article:
> With previous research in the United States having proven the accuracy of GRACE in detecting groundwater [researchers] were looking for a region where they could apply the new technique.