[JKT]

Well there's the problem - with the exception of Visio, all those apps are first and foremost illustration apps and not diagramming apps. OmniGraffle is first and foremost a diagramming app and is not intended to be an illustration app. Personally, I wish it was to a much greater degree, but there are arguments for and against feature creep.

Btw, the PDF itself is not editable, but double clicking it opens the LinkBack original version of the image which is editable.