The reason is that those who do this are educating potential competitors how to compete with them direct.
I have been exporting to China since 1978, and importing from China since 1987, so I have a good idea how things work. In that time I have seen many cases of Chinese companies opening branches in Western countries and competing with those people who were previously buying from them and importing their products.
It will be so much easier and less costly for Chinese companies to ship their goods to FBA. Now they don't have the huge expense of setting up an office overseas. Instead they have Amazon.
There are plenty of prepping companies that will receive the goods for you and do whatever is necessary before shipping them on to FBA. The small cost would be a lot less than losing your entire business to your supplier.