US is willing to stop nuke testing, but considers sub 1-kilon nuke tests in secret underground containments exempt from this "testing-ban". It is easy to stop testing after you've already done 1000 tests, in earth, water, air and space, while also leaving the option of secret sub-kiloton testing open. Why deprive others of testing data, they also have curiosity and desire to learn and explore. There are innocent scientists in other nations too, that are not American!
