The music industry is predicated
on the album format. While this may be redundant to many, as I have expressed the opinion before, ALL selections on an album are compensated the same in terms of the mechanical royalty paid by the label to the writers and publishers of each
of them. Thus filler material royalties are paid based on the sales of the album generated by the one or two hits on it.
Naturally, writers such as Metallica or their relatives/friends whose "B" sides appear on albums are loath to forego the album format. Often, producers and labels manage to get "their own" compositions onto albums as a condition of making and distributing the recordings.