Inkscape on OSX copies rasterized images from SVG
Here someone had the same problem.
They solved it by turning off X11's Pasteboard Syncing:


  Disabling ‘Sync PasteBoard’ Functionality within X11 (Which is
  Included in XQuartz App for OSX Mountain Lion) solves the problem of
  converting vector object to Bitmap after pasting the same object in
  another SVG file.
  
  Do This (Go to): X11 > Click on ‘Preferences’ tab > Choose
  ‘Pasteboard’ tab > Uncheck ‘Enable Syncing’