I have iTunes 12.01.26 on Yosemite. I click on my name on the top right and click "Purchased", then "Apps".
I cannot drag the app to the trash. Right clock does not work either. I can click on the app and hide it. Apple must have updated iTunes as of December 2014 to prevent purchased app being deleted.
The reason I want to delete apps is because I have some really, really old apps that I no longer use (think from iPHone 3GS days). It is now cluttering the list and it is really hard to search for purchased app when I am on my iPhone. Not sure why Apple is preventing users from deleting the apps from the Purchased list.