Apple - 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’

I solved this problem by importing another svg file into the current one.

File --> Import... --> Select "All Vectors" (or "*.svg") in the filter selection menu.

This copies the entire content of the imported file into the current one in vector format.