Throughout history, even before it became a global superpower, the United States fought to defend the freedom of the seas. If Iran forces the US to accept an extortionary toll on the Strait of Hormuz, it will be a great blow to the US-led order that allows trade across international waters without heavy fees enforced by deadly power.
Control of the Strait gives Iran control over the price of oil. If they want the price higher, they let fewer ships through.