I can't provide any documented proof on this, and the info I have is a decade old and second hand, but there was at one point a culture of skimming, kickbacks and cost cutting that pervaded China. The stories I've been told had to do with printed goods, books, board games, card games, etc related to the table top gaming industry (think: Dungeons & Dragons, Magic the Gathering, HeroClix, etc.)
In general, many of the factories in china were perfectly capable of producing products on-par or even in some cases exceeding the quality of the products made in America or Europe. When large companies would spec a product, they would sometimes send representatives to China to evaluate the production line, check quality on pre-production samples and sign off on the production of however many units they needed (be it 10k or 1m units.)
The problems would generally start as soon as the factory would re-order supplies. If the U.S. company provided material specs to the factory and allowed the factory to source the materials, than as soon as the factory needed to resupply they would send out the materials for a rebid from their suppliers (or if they had a preferred business partner they would just go direct to them) -- this would often result in the factory either getting the materials for less and keeping the difference for themselves, or getting kickbacks of one type or another -- the problem was that sometimes these new materials would not actually meet spec and would result in lower quality product. Since the U.S. company would normally not have one of there people "on the ground" these sub-standard materials wouldn't necessarily get caught and would make it into the U.S. supply chain and out to retailers. Now sometimes, these would get identified later, sometimes they wouldn't. If found, sometimes the U.S. company would switch factories -- but generally the costs of moving production would out weigh the benefits and they would get "reassurances" from the factory that they would only use approved vendors for materials in the future (which would sometimes stick and sometimes not.)
The largest companies keep a quality team in-place, on the ground, locally in China to make sure these things don't happen -- but many companies find it cost prohibitive.
Another anecdote was an NPR story I heard last year that had to do with Apparel manufacturing. The problem that industry would have is that they would get production samples from a factory and they would be perfect. The factory would start producing the product and suddenly the product would get popular. The manufacturer would contact the factory and ask if they can ramp up the production on their next run so that instead of 10k units on the next shipment they need 50k units. The factory, not wanting to loose the business, would say "sure no problem" even though they knew they couldn't possibly produce more than 20k units in the given time frame. The factory would then subcontract with other factories to produce the extra and those factories would then produce sub-standard product that would end up mixed in with the good stuff.
This kind of behavior is not unique to China, or even to just a few regions -- this kind of activity happens world wide, it is just more pervasive in China (or at least it was a decade ago.) It's the responsibility of the manufacturers to find reliable factories that can produce on-spec, and to spend the resources needed to make sure the product ships on-spec. It just happens that when you own your own factories, or when your employees don't have to travel halfway around the world to do Q/A, than it's more likely you'll keep a closer eye on the results.
Not much point to arguing about all of it though -- perhaps TLG decided that the collectible minifigs were less likely to be "played" with in the same manor as the figures from their normal sets, and as such they could cut some corners with the particular quality of ABS used, the maintenance & quality of the molds (are the mold lines more visible on the CMFs?), etc.