If a referendum were held tomorrow in America on whether or not America should join the EU, you'd vote against joining the EU. Overwhelmingly so. And yet your politicians are begging the British to remain in the EU, an organisation they themselves wouldn't want America to be in.
As for your argument that Britain is already in the EU but America isn't, this is addressed in the OP:
Perhaps you think my analogy unfair? We are already in the EU, whereas I am suggesting that America joins the — currently fictional — AU. So what? Surely the decision is identical. If the AU/EU is worth joining, then it’s not worth leaving, and vice versa. Perhaps you feel the cultural and economic differences between Seattle and Tegucigalpa are greater than between Manchester and Athens. I don’t agree. Perhaps you think it unrealistic to expect such a big country as America to subsume itself into such an arrangement. Well, Britain is vastly bigger than many very successful, independent countries and has the fifth largest economy in the world. America could expect to boss the AU far more than we get our way in the EU.
Perhaps you think America should be more concerned with building free trade and good relations with people on other continents, rather than the countries that happen to be next door: that is, with China, Russia, Brazil, Europe. In which case, don’t you think the same is true for Britain? Silicon Valley has benefited from a flow of talent from the Indian subcontinent — precisely what we have denied our creative industries here as we struggle to control immigration overall but are not allowed to restrict numbers from one particular landmass.
There is a serious point here. Most Americans I know think Britain would be mad to leave the EU, but that’s because they think the EU is like Nato or Nafta or the Organisation of American States — a club of nations bound by a treaty. They think it is a trading bloc. They do not appreciate that it is a common government, run by a common bureaucracy and answerable to a common court system. Once you explain this, by using the analogy I just used, they get it immediately. They would never join the AU in a million years.
And then pause to consider the irony of America, a country born in rebellion against being governed by others through a democratic deficit, lecturing the British on how we should stay inside the EU. The chairman of Conservatives for Britain, Steve Baker MP, had this to say about John Kerry’s remarks:
‘I refer Mr Kerry to the US Declaration of Independence. We will do peacefully at the ballot box that for which his nation fought a war of bloody insurrection. If the USA must express a view on the UK’s right to the separate and equal status among the nations of the world to which many of us feel entitled, perhaps they might consider whether they wish to discuss their back taxes.’