Practically no areas of the world are completely uninhabited. Even the Sahara desert (where the French tested their weapons) has inhabitants. But the population density of these areas is extremely low.

I don't know the specifics of Beria's choice, but the US's choice in testing at Nevada Test Site (with plenty of cities downwind) was dictated by the need for a test site that was not on an island in the middle of nowhere because preparing tests on distant islands is expensive, difficult, and dangerous. They also underestimated the health effects of downwind fallout of this sort considerably. To the Soviet case we might add that a callous disregard for local populations was pretty common to all of their major industrial and certainly nuclear endeavors, at least during the 1940s and early 1950s.