It really all comes down to people that do not understand the difference between diethylene glycol and propelyene glycol, the later by the way being a regular ingredient in many foods you consume.
While these incidents may not be intentional poisonings, some of them certainly involve a callous indifference to the possibility of harm. The replacement of heparin by oversulfated chondroitin sulfate was obviously a very sophisticated fraud. Similarly the addition of (nitrogen-containing) melamine to raise the apparent protein content of wheat gluten was a fraud.
As for diethylene glycol, it appears that it was substituted for the (more expensive) glycerin rather than propylene glycol. "Glycerine and diethylene glycol are similar in appearance, smell, and taste." - Wikipedia.
Indeed there was fraud involved in the two cases you cite. But where is the quality control of the companies that are purchasing these items? They have a responsibility to know what they are buying.
As for diethylene glycol, it appears that it was substituted for the (more expensive) glycerin rather than propylene glycol. "Glycerine and diethylene glycol are similar in appearance, smell, and taste." - Wikipedia.
It is definately callous indifference as you point out above. It is also called understanding the conditions in which one is doing business and adjusting for them. While a U.S. buyer may never think someone would do such a thing, that is his job to know purchasing conditions.
U.S. companies are already getting goods on the cheap in China. What, they want to save on quality control also! Did the U.s. company that bought the toothpaste have anyone at the factory while the toothpaste was being produced? that may sound like an odd question to some, but it is not.
Anyhow, good detail you bring. I think we agree.