You misunderstand. Chinese users in China connect to Dropbox and Google Drive servers outside of China, where nothing is available to the Chinese government. Non-Chinese users in China connect to Chinese iCloud proxies in China, which handle data that has already been decrypted using the storage key, which remains outside of China, so anything the user accesses is available to the Chinese government. Chinese users in China have their iCloud data stored in China, where it is all accessible to the Chinese government at any time. Aside from that, there is no meaningful difference in privacy among the services.