Licensing?

CNN has a thing about Matthew Weiner changing a song from the season premiere of Mad Men (Dusty Springfield doing "The Look of Love" [Bacharach, not proper, superior 1982 Fry/White composition]) because critics pointed out that the song came out seven months after episode's established date-of-fictional-happening.

So why then, did the first episode of the series (when I saw it) feature "The Great Divide" by the Cardigans and some other episode show all the Mad (Wo)men getting ready in the morning to "The Infanta" by the Decemberists?

And when will the Decemberists stop?