DMA has not been used against EU tech companies because US tech companies are clearly the market leaders in the area the DMA is concerned with. The DMA exists to make sure that companies (from the EU, US, or elsewhere) comply with EU regulations regarding privacy, tracking, and consumer rights.
It's not a "tax" on US companies, it's just that US companies don't bother to comply with the regulations that apply in the EU, and thus get fined.
It's not a "tax" on US companies, it's just that US companies don't bother to comply with the regulations that apply in the EU, and thus get fined.