What frigging right does the US have to impose sanctions on another country (outside of the UN)?

When I read news stories such as this, this, this, this, this, and this, my blood boils and turns into lava.

Can someone please explain to me what fugging right does the US have to unilaterally impose sanctions on another sovereign state? Why are these sanctions passed as "laws" in the US Congress, instead of the UN headquarters? Who the hell is this Barack Obama dude?

(source)

Laughably, how in the frigging hell can the United States pass a law in its (troubled) homeland which bars other countries from exporting refined petroleum to Iran (among other things)? And why are these "other countries" so impotent as to accept these laws and sanctions? Why is the UN impotent as well?

No matter how much I try, I fail to understand (and tolerate) this.