If the satellites are in polar orbits, they pass over the poles every 1.5-2 hours, if they are in LEO. You might have to wait a day or more to get a shot at them from any particular mid-latitude longitude. The pole provides an obviously advantageous base of operations for this reason. Whether this justifies the supposition that the Chinese motives are other than purely scientific, I couldn't say, except I don't think the Chinese could recognize a purely scientific motive if it bit them in the ass.
Every knows the Chinese are there because they are looking for the Nazi UFO Base! /s
http://www.germanufochatter.com/Nazi-South-Polar-Base/