There's two different ways of looking at this issue. One belongs to Charles that is something deep rooted within him that cares deeply about his fellow human beings. Then there's The Prince of Wales going somewhere at the request of his government, representing his monarch and his government and what their stance is on the issue. With this trip, we'll be seeing The Prince of Wales rather than Charles the man. Charles may have stood up for human rights with China but I would bet my last hard cream soda that it wasn't on an official visit to China at the behest of his monarch and government.
In his role as The Prince of Wales, he sometimes has to do things he might rather not do. It will be the same when he is King.