The Open Door Policy is a foreign policy. It allows any nation to access China, but none of those countries can control China. The concept of an open door policy is that any nation can trade with the nation that holds the open door policy.
If America adopted an Open Door Policy, it would be certain to make the country prosperous and lower the prices of many things for its consumers. Just because it is good for the United States, does not mean that it is best for the rest of the world. America places tariffs on countries, because they are not complying with what the rest of the world wants them to do. The tariffs on these other countries make their economy plummet, but that does not mean that the United States are unaffected. These tariffs also hurt America, but much less. The United States are putting so many tariffs on other countries that these small effects are starting to play a major role in the falling economy. If America were to take these tariffs away, it would benefit greatly, but by taking away the other countries punishment they would have no reason to listen to the rest of the world. If these countries are not punished and America is supplying them with a steady flow of cash, then they will quickly develop and become a threat to everyone else. America would benefit greatly from an open door policy, but the rest of the world would suffer as a consequence. They would suffer because America is such a super power that if it took away its tariffs, then the affected countries would have enough money that they would be able to become a threat to the rest of the world.