The fact that the AV is not under copyright in the US is meaningless as an argument either in favor of or against it as a translation. It just doesn't prove anything other than the fact that the AV was published before 1923 and like ALL other books published before 1923 it is public domain in the US.

If a person wishes to argue in favor of the AV, there are a lot of very potent arguments that can be made. This just isn't one of them.

In fact, the Revised Version of 1885 and the American Standard Version of 1901 are also not under copyright.