Yes it is by design. HKG/MFM/TPE flights are grouped together with international flights in mainland china, and some flavor of cabotage rules apply.
Also, under no situation would I seriously consider TFU-HKG-PVG vs CTU-SHA, unless I am reviewing a product, flying for fun, or something of that nature. For an MR, I think TPE is better, so are many SE Asia destinations with cheap business class fares