Some organisations are granted special status - for example, CERN is built straddling the frano-swiss border and can issue its own diplomatic vehicle registrations. Not to mention things like the UN building in New York.

If for some reason the US thought ICANN shouldn't be subject to US law, they could do something similar. Being inside a country's borders doesn't always mean being subject to its laws.

Of course, the corruption in the .org selloff doesn't make me feel ICANN needs less oversight. And it's questionable whether the US can grant independence it cannot revoke, at least from an international realist perspective.