When China applies tariffs against the US, the result is that China imports the goods it needs from other countries. For example, now that US soy is prohibitively expensive for Chinese importers, they can buy from Brazil (who China is not applying the same tariffs against).
In the case of the US, the US importers cannot switch to anyone else, since it decided to go full trade war against every single country.
This is doing a lot of heavy lifting here. The US can say it's tariff policy is whatever it wants but the administration is under constraints. If there was some extremely compelling data that the economic effects caused by the retaliatory tariffs would cause an overwhelming democratic victory at the midterms that would have a much bigger effect on policy than the formula for tariffs the administration wrote down. I am not predicting this definitely will happen but it is the kind of thing I am sure people are considering when they are imposing retaliatory tariffs.