Well, you see, because of reporting problem with too many of the stations, they don’t know the locations of many stations that closely. Urban areas comprise only about half a per cent of the Earth’s land surface. Sure, if there was good data, they could do anything they wanted with it. The challenge here is to extract useful knowledge from really noisy data.
Point is, when it comes to the Urban Heat Island effect, if they have a known non-urban population to compare with, they can eliminate the effect, or at least put a good rough size to it.