There is no valid reason for them to have the keys
They don't need the master key unless they want to change things, and they are not supposed to change things, so they shouldn't have the keys.
Take temptation out of their reach.
There is no reason to let USA decide which server resolves .cn .jp .uk or any other top level domain. They change almost never, and each zone could (and should) keep their own top level list separate from the US. It's basic security, you don't become reliant on one provider for anything.
Changes to top level domains should be approved by authorities in each zone. So should you wake up tomorrow and find .eu suddenly is resolved by a server in Yakima or Virginia, you can decide to reject that change of DNS and your internet zone continues normally.
Better to let changes propagate slowly rather than hand control to the fruit loops.