The official reason EU added tariffs on Chinese cars, was that Chinese EV manufacturers receive unfair government subsidies. This allowed Chinese cars to be priced lower than European cars.

So it was to protect EU car manufacturers (which are still struggling a bit with EVs).

I don't think it was US "soft-power". However, the EU may impose tariffs on US produced cars as part of the coming trade-war, which may hurt Tesla in future (or maybe not as they also produce Tesla cars in Germany).