I have had the opportunity to have a product manufactured for me in China, though made of metal not rubber. I commissioned large cloisonne grill badges for a British sports car and was unable to find a manufacturer in the USA. The Chinese company that I ultimately used was a family business doing this specialty work for over 125 years.
It took oversight of each batch that they turned out, from prototype to finished product, to maintain the quality that I needed. They didn't like it much when I returned an order of 50 pieces but after the second return all the future runs were spot on.
Rubber production is probably more complicated due to the nature of the rubber itself but it ultimately comes down to not accepting out of spec products from the factory.
In the case of our trucks I don't know who is commissioning the production of the parts. It seems doubtful that the individual retailers are ordering directly from the factory but the bottom line is whoever is paying for the production has the responsibility to insure the parts are right.
In my experience the factory was very responsive to my demands and once we were all on the same page things ran smoothly.
One final thought is that the factory seemed surprised that I was willing to pay more to get the quality I needed. They are used to producing things as cheaply as they can, as big businesses grind them down to nothing on the price they will pay.
The old saying 'you get what you pay for' is as true in China as it is in America.