Why is US blocking hormuz straits then?

The US is is not blocking the Strait of Hormuz. There don't appear to be any US warships even in the Strait at the moment. What the US is doing is enforcing a partial blockade against Iran, largely in waters southeast of the Strait. We can argue about whether this is a good policy but let's not make things up.

https://www.centcom.mil/MEDIA/PRESS-RELEASES/Press-Release-V...