Apparently they think their albums suck and are afraid that if people can buy individual songs then they'll never make the mistake of buying the entire albums, thereby making Metallica poor.
I really like the iTunes music store but if I want an entire album I still buy the CD. If I just like a single song and not the rest of the album that's all I'll buy. If the only option I have is to buy an entire album to get one song I like (and twelve I don't) then I just won't buy the album. Groups like Metallica need to realize this is adding sales, not taking them away.