My interpretation of the restriction against importing milk powder to the mainland includes the article's mention that prices were rising in HK, which could be a problem for a lot of locals there, who can barely pay rent. Considering the economic benefit to HK retailers because of individuals, not dealers, who cross the border from mainland to shop in HK for milk powder, it seems that the incoming money could be a good thing and should be encouraged. Still important to consider that in the immediate sense, it might actually be causing deprivation for monetarily poor locals in HK.
However, when you consider the totality of the melamine scandal in China, USA, New Zealand, etc., in the context of obvious control mechanisms in those countries and the EU to prevent people from drinking the healthiest, freshest, safest varieties of milk and milk products, I really wonder if the restriction against importing healthier milk powder to the mainland is not only part of a trading agenda, but also part of a health agenda.
For example, Nestle, a huge Swiss food corp., will not allow importation of its Swiss milk powder to North America, and it was Nestle who was at the center of the melamine scandal in the USA that broke in 2009, leading to the immediate change in the FDA's re-classification of melamine as a toxin with zero safe levels, to allowing enough that a person my size/weight can "safely" consume 5 GRAMS (table spoon) per day. That regulatory change was made without public comment or testing, etc. I saw it as an edict from Nestle, your interpretation may vary.