Janet Yellen spoke to NPR and cleared up the reasoning on this.
President Biden announced tariffs on roughly $18 billion of imports from China. They're very carefully targeted at sectors that we're supporting through legislation that President Biden passed with Congress, the clean energy sector, semiconductors, sectors where we consider it critical to create good jobs.
We're seeing massive investment in manufacturing in these areas. And we think it's very important to protect our workers and our firms in these strategic sectors from the kind of dumping that results when China develops massive overcapacity in these areas.
Yellen didn't come right out and say it, but they are afraid the green businesses in the US that are getting big government grants still won't be able to compete and will go out of business. The actual words were "protect American jobs".