Actually, it's not. Just because a song may have be written for a particular album, at the same time, if she doesn't use it can always be pitched to another artist for them to record (and as such, I believe this becomes another income route should she wish to do that).
Sadly after the 'leak' of A Place I Wanna Be' a whole load of her tracks were pulled that we'd never heard of before. Still registered as having been written but no masters
At least, not publicly. It was only a demo though and as I think we probably know, Amy wouldn't want anything out there that she wasn't truly happy with (I mean, look how long Under Stars took....)
Besides...maybe in a few years we could a rarities & demo album...think of it like that