Yes, the reason for that is that when warping images, you are pulling intensity values from the orignal space onto known positions (voxel centres) in the target space. So you need to know where a voxel in target space maps to in original space to figure out where to fetch the intensity value from (and an interpolation method to fetch values at arbitrary locations).
With streamline data, we have vertices in the original space, and we need to figure out where to push them to in the target space, So we need to know where a location in the original space maps to in the target space. As Örjan says, this is the inverse of the transformation required for images.
Hope that makes sense…