Pupdog, that indicates that the entire album has not been made available for sale on the iTunes Store, only some tracks. It is not common, but it can happen when one or more tracks have different ownership/rights arrangements.
Because the record company requires you to purchase the album to have the song.
The Mark - album - was created in 2002.
Mark Heard - album - was created in 1975.
Mark of the Beast - album - was created in 1981.
Mark McGuinn - album - was created in 2001.
Mark Williams - album - was created in 1975-06.
Mark Hollis - album - was created on 1998-02-02.
Exclamation Mark - album - was created on 2011-11-11.
Mark Chesnutt - album - was created on 2002-05-21.
Outlaw - Mark Chesnutt album - was created on 2010-06-22.
Peace at Last - Mark Read album - was created in 2008.
Wings - Mark Chesnutt album - was created on 1995-10-03.