While, yes, caveat emptor, I have purchased a number of devices which are based on the Arduino open design. I have not yet had a problem.
Remember that, if you buy more expensive, prominently "Arduino" branded parts, from a third party, you also have no guarantee that they are quality controlled by Arduino. Even "genuine" electronics parts are usually manufactured in China, often in the same factories.
To answer your question, there are a number of ways the costs can be kept down:
Lack of unions, lack of labor laws, economies of scale, markup by the "official" companies, lack of development costs (remember, Arduino is open hardware, copying is allowed), cheap labor, tax, transport costs (there is some trickery in some countries, including Hong Kong, which makes shipping essentially free), shorter supply chain, cheaper components, smaller marketing budget.