Cannot move purchased music from iPad to MBP

This is a really stupid question and I apologize for the ignorance, however I am wondering why I cannot move songs purchased from my iPad to iTunes on my MBP. When I tab to the Music section in iTunes and try to sync, I get the message warning me that all music on the iPad will be replaced with music from iTunes. I don't want to replace, just keep things the same. What am I doing wrong?

thanks