Even if they end up buying the same product overseas as they would from the shop down the street, the main problem is that the quality checks for the local market is fraudulent or non-existent. Mainland Chinese cross the border to HK on a daily basis to buy and bring back products made in their own backyard, they're basically taking advantage of the checks that the HKG government has in place for imported goods. I have a friend who worked in the HKG Dept of Health, specifically food safety, and she would tell me about the regular meetings and exchanges she has with the Chinese government in setting quality standards and implementing monitoring mechanisms for foods coming into HKG.