There are certain albums on the iTunes Store that contain one or two particular songs that everyone wants, plus a bunch of songs that most people aren't so interested in.
The way that's often handled is that the most popular songs are labeled "Album Only," meaning that you can't buy them individually, only as part of the whole album.
This is, of course, annoying to people who only want the one song. But it's remarkable how many people post album reviews for those albums that say things like (paraphrased) "This is ridiculous! Why make the good songs Album Only? The good songs are the ones I want, so you should make those available individually even if you make the rest of the songs Album Only!"
It's possible that the people who post such comments are just expressing themselves poorly, but they really give the impression that they're missing the point. Of course it's the popular songs that are album-only! The point is to force people to buy the album, not just the one or two best songs!
Various reviewers have said that the record label (not Apple) gets to decide which songs are album-only; don't know if that's true, but it sounds plausible.
Of course, it may still be worth complaining about; it's possible that the volume of complaints may some day have an effect. (Though in several cases people have now been complaining about this for years with no change.) But my recommendation is that if you're going to complain about this, you should do so with an understanding that it was an intentional decision, not some kind of accident or mistake. The record label is not going to suddenly say "Oh! Of course! That track is the only one that everyone wants, so we should stop making it album-only!"
Btw, I'm not defending the idea of album-only songs; I too would like to buy just those songs. (Though I often buy whole albums when I like roughly more than half the songs on the album.) (My system for deciding is more complicated than that, but that gives the general idea.)
I think I first encountered these misconceived reviews on the Toy Story 2 soundtrack album, where Sarah McLachlan's "When She Loved Me" (definitely the standout song from the movie) is album-only. Most of the (currently) 65 reviews of the album say things like "It is the only song on the album to be so restricted despite the fact that it is far and away the most popular and desired song on the album." (Emphasis mine.) Well, no--it's not "despite the fact," it's because of the fact.
Most recently, I saw the same kind of reviews for the Music & Lyrics soundtrack album, where a remarkable ten out of the twelve songs are album-only, including (of course) the two that everyone wants: "Pop! Goes My Heart" and "Way Back Into Love." In this case there are over 1500 reviews, and although I didn't read very many of them, I would guess that roughly 80% of those include complaints about the album-only thing.
I was almost ready to buy the Music & Lyrics whole album, but then I saw that it cost $11.99 instead of the usual $9.99. So I checked the new Amazon MP3 store, and found the album in MP3 format for only $8.99, though over there only one of the songs on the album is available individually. I decided I liked the album enough to pay $9 for it, so I bought it. Downloading went smoothly (this was my first time purchasing from the Amazon MP3 store), and now I'm listening to "Pop! Goes My Heart."
Of course, anytime you're online and you want to listen to that song, you can go watch the very entertaining video on YouTube for free. By far the best '80s pop video made in the 2000s.