Is there an explanation for the US equity market’s reaction to Trump’s protectionist announcements?

Each time Donald Trump imposes tariffs on imports from China, US share prices fall (September 2018, May 2019). Yet for the United States, tariffs on Chinese imports are a zero-sum game: the rise in import prices is completely offset by the transfer to Americans of the amount of the tariffs (which are redistributed). For share prices to fall, investors must therefore fear retaliation from China, which can actually be seen in the fall in Chinese imports from the United States.