preprints are free but when they are added to an issue of a journal, that issue might not be free. so journal X agrees to free preprints (rough drafts) and then

preprints are free but when they are added to an issue of a journal, that issue might not be free. so journal X agrees to free preprints (rough drafts) and then charges for the actual issue. if the library drops subscription to the journal, maybe the journal/society/publisher pulls the library's permission to use the preprints.