Because there's obviously some activity going on in it, for instance the logfile named ReportingEvents.log is locked and in-use, also in some of the files and folders they are either created or updated when you are online which may have to do with Automatic Updates - I don't know for sure since I haven't searched for it.
However a locked and in-use file and files and folders that are automatically changing and updating on a daily basis would suggest that folder shouldn't be messed with by a cleaning program or an end-user.
Dear Moderator;
The \Windows\SoftwareDistribution\download folder is used by Internet Explorer to do automatic updates. Sometimes, this folder becomes corrupt --just like the ..\TEMP folder becomes corrupt, and needs to be cleaned out. However in order to do this, you need to stop two Windows services, rename or delete the folder, then restart both services. The steps required to rename the softwaredistribution folder are as follows:
At the command prompt, type the following commands:
uninstall-folders under c:\windows\softwaredistribution\download
in CCleaner
Posted
Dear Moderator;
The \Windows\SoftwareDistribution\download folder is used by Internet Explorer to do automatic updates. Sometimes, this folder becomes corrupt --just like the ..\TEMP folder becomes corrupt, and needs to be cleaned out. However in order to do this, you need to stop two Windows services, rename or delete the folder, then restart both services. The steps required to rename the softwaredistribution folder are as follows:
At the command prompt, type the following commands:
net stop wuauserv
net stop bits
ren [or del] %systemroot%\SoftwareDistribution SoftwareDistribution.old
net start wuauserv
net start bits
After the Windows services are restarted, a new softwaredistribution folder will automatically be created.
Please refer to Microsoft TechNet article 934562, or go to http://support.microsoft.com/kb/934562.
Hope that helps...