Often, Wikisources use files that are not PD in both source country and the US, and therefore cannot be hosted at Commons, but can be hosted at the Wikisource as they may use one or the other only (e.g. enWS uses only US copyright).
Copy-uploading is a convenient way to transfer files to wikis from upstream sources like the Internet Archive, etc. This will also allow IA Upload to upload to Wikisource if needed.
As mentioned in the Gerrit review for T287241, wgAllowCopyUploads is false for Wikisources.
This should be enabled, and wikis can then assign upload_by_url rights as they see fit (on Commons, this comes with the User group).