When omitting versionable aspect, there is no way to 'overwrite' the document. The only way to load a new doc is to 'replace by new version', which implicitly makes the document versionable.
I would expect that a 'non-versionable document' could be overwritten.
Do someone has informations / suggestions on that topic ??
When using the "share" interface to upload, the versionable aspect is turned on automatically when a file is updated. The designers seem to want all content versioned when uploaded through "share". This may be what you are experiencing.
The "alfresco" interface leaves versionable alone, so that user settings and rules can work consistently.
I personally don't like the way that "share" messes with versionable…