Why does the New American Bible translate Luke 1:28 the way a Protestant would?

Because they are like Protestants; that is why. These liberal scholars will look for the slightest excuse to go liberal, and there are a lot of liberal scholars in Catholicism, at least ninety percent in all the Catholic seminaries and universities, which means they will not give you the traditional understanding of Luke 1:28. They render it 'highly favored one,' never 'full of grace.' They could at least put 'full of grace' in the verse and add a footnote explaining that it comes from the Latin gratia plena and that the Greek could be translated another way, and then the reader would understand; but they do not, and it is sad, very sad.