CBS marketing ploy on all of their recent Woodstock albums - they included 1969 studio album by the same group (Airplane, Janis, Sly, Johnny Winter, Santana). In all the other cases, that pushed it from a 1-CD set to a 2-CD set, and they left off any bonus cuts which had come with other latter-day CD versions of the studio albums, but the Airplane set would have been 2-CD anyways, and only a $19.98 list, so while including 'Volunteers' is goofy, it's no real loss.
... and the major labels wonder why their business is going down the tubes.