Explanation/Reference:
Ref: http://technet.microsoft.com/en-us/library/dd379484(v=ws.10).aspx
By default. Active Directory Recycle Bin in Windows Server 2008 R2 is disabled. To enable it, you must first raise the forest functional level of your AD DS or AD LDS environment to Windows Server 2008 R2, which in turn requires all forest domain controllers or all servers that host instances of AD LDS configuration sets to be running Windows Server 2008 R2. After you set the forest functional level of your environment to Windows Server 2008 R2, you can use the instructions in this guide to enable Active Directory Recycle Bin.