Agreed. Mad Men premiered in 2007, well into the HD era, and was already composed for 16:9. The issue here is that the 4K "remaster" seemingly wasn't mastered at all, just a re-scan of the original negatives at a higher resolution.

That said, I think letterboxing is a better choice than trying to recompose a shot for a different aspect ratio if it wasn't accounted for initially (eg: The X-Files). And even then, the shot selection and composition would have been different had the aspect ratio been initially different (like TXF's 16:9 versions leaving the edges of the frame largely unused).