What gives the US proper jurisdiction?

The company that chose to operate within the US jurisdiction, in these cases, by owning the vehicles to doing the transport.

Iran did not agree to be sanctioned.

What kind of schoolyard logic are you working with here? Do you really have no idea how geopolitics works? No country has to have permission to sanction another. It is a choice one country makes to no buy from another. There is nothing preventing Iran from selling its oil to China. They're just not allowed to do it with anything that is owned by the US government, US companies and those countries that choose to follow the same sanctions.

Nor do they have to adhere to a law made in the United States, unless they agreed to it internationally.

They absolutely do if they're using something owned by the USA, in this case the tanker itself.

My argument is sound.

Your argument is naive, willfully ignorant, or maliciously in bad faith.

Other countries don’t have to obey US law, unless they agreed to that law.

Indirectly Iran agreed to it with the use of a US owned tanker. Why did they think they could do that when it breaks US law?

This isn’t difficult.

I agree it isn't difficult. Don't want to be bound by US rules and law? Don't use US owned property, operating in US waters, use US banking systems, or any of the other countries that choose to follow US sanctions against a country. See how easy it is?