Cursors require maintaining a fair bit of state. Implementations may also do optimizations such as pre-fetching records. This state can only be completely released when either the cursor hits the end of its range or both the request and the cursor have been GC'd (since there's an implicit bidirectional dependency), or when the transaction ends.

It might be nice to expose a close() method on cursors to allow web apps to signal that a cursor can be tidied up prior transaction closing.