Some artists have given in to the iTunes Store juggernaut (Metallica, The Beatles, AC/DC), others still hold out (Garth Brooks, Def Leppard, Bob Seger), and some can't decide (Kid Rock). But what they all agreed on, at least at some point, was that they did not want their albums sold off piecemeal. Why? Because, as Garth Brooks put it:
I don't want singles 'cause you make an album for a reason, an album is a reflection of who the artist is at that time.
Sorry Garth, as much as I love your music (and I do, Much Too Young, for example, has a permanent spot on my playlist) this excuse is bunk. And I'll give two good reasons to back up my assertion that the whole "album as a reflection of an artist" thing is bunk.
These compilations do not represent who you were at the time you recorded the albums because the tracks span many years, and I suppose some argument could be made for hand-selecting a cohesive career retrospective, but these albums are simply collections of your most popular songs. And this isn't to pick on Garth Brooks exclusively, Bob Seger and Def Leppard are just as guilty.
Don't make some artistic stand, just call it what it is, Apple uses iTunes content to make iPods more attractive because they want the money that selling those iPods brings in. But artists also want the money whole albums bring in. It's all a money game. And right now everyone's losing.
Update: As Paul Rumens notes in the comments, Def Leppard are arguing with the label, not Apple.