There isn’t a rule, it’s more of a guideline, and I know for certain that debates over which to pick do happen internally. But it doesn’t matter. Almost no one actually notices that some services are called Amazon X and others AWS X. Of the people who notice, almost none of them actually know what the difference is supposed to be. Of the people who know, almost none of them care, not least because it’s applied haphazardly. So it really doesn’t matter.