It is and should be the decision of the owner of the song to decide how it is going to be marketed – the owners of the songs on the compilations choose to include them, the owners of songs traded on illegal file-sharing networks generally don’t.
There are actually six big “labels” (well, really five and a half). The term is a misnomer – technically a label is something like Interscope, Bad Boy, Blue Note, etc. – a line of recordings put out by a single staff and its stable of artists. However, most labels are associated with one of the big six, five of which are really publishers. Many labels (such as Capitol Records) started out as independent companies which were since purchased by one of the big publishers. The issue is further confused by the fact that most of the publishers not only have several associated labels (some wholly owned and many co-owned with producers or stars) but also release albums in their own name, acting as their own label.
The real big five are Universal, Warner, EMI/Virgin, BMG (Bertlesmann Music Group), and Sony. As you can see, each one of these publishers is part of a major entertainment conglomerate – Vivendi/Universal, AOL TimeWarner, Virgin, Bertlesmann, and Sony. Until a couple years ago, these five publishers (actually, I believe EMI and Virgin were not yet merged then, so there were six) controlled a huge percentage of albums sold. However, the teen pop craze came out of an independent label, Jive Records. N’Sync, The Backstreet Boys, Britney Spears and others were all signed to Jive which gave it (and its parent company, Zomba) a market share orders of magnitude bigger than any of the other independents, so it really serves as a sixth member of the club, despite being distributed (but not owned) by BMG. However, now that it appears the teen pop craze is both cooling considerably and that other labels have their own teen acts (Destiny’s Child is on Sony and Avril Lavigne is on BMG’s Arista Records), it’s an open question whether Zomba is going to be as important in the future as it was from, say, '98-'02.