Total speculation here, but probably because common law trademark would have to be proven in a court of law. That is, he would have to sue the Chinese company,

Total speculation here, but probably because common law trademark would have to be proven in a court of law. That is, he would have to sue the Chinese company, present evidence like newspaper clippings or testimony from customers, win, get a judgement, and present that to Amazon. Until then, Amazon looks in the USPTO database.