It isn't, though. Getting a ringing response is enough to validate a number, and someone who isn't making the effort to check contact lists against do-not-call registries won't be making the effort to track numbers that do not answer, either. In any case, RTNR to such people simply means that there's no-one to answer right now and is no reason to discard a number.

See https://news.ycombinator.com/item?id=16871131 and https://news.ycombinator.com/item?id=16873537 for what's actually happening.