That's not Bandcamp's fault. Whoever listed the album configured it that way. There are settings for artists (or labels or whoever) to allow people to purchase single songs, or require that they buy the entire album.
I've bought plenty of single tracks from albums. A few of them as recently as this past weekend.
Either I didn't explain it correctly or you didn't read what I said entirely.
If an album has a free song or single, you cannot add the free song to your library unless you purchase the whole album. Even if you are allowed to purchase individual songs on the album.
In general they don't add free stuff to your library, they just let you download it. I can't recall any free album that could be added to my library (I don't personally mind, I just download stuff anyway). There used to be a $.50 minimum to add something to your library but I think they might have dropped or reduced that at some point (you do need to pay something, though).
I've bought plenty of single tracks from albums. A few of them as recently as this past weekend.