When my mac is backing up on time machine I get Threats messages ...why ?

Hello PaulM,
 
Welcome to the Webroot Community,
 
Webroot recommends changing your settings so that you do not scan mounted drives with WSA. In some cases, Webroot will detect a threat that is located on your backup, such as Time Machine. If the file are in the backup, then they cannot hurt your system. You would have to restore the files from the backup to get them on the system, and at that point the Real Time Shield in Webroot would find and remove them. Even though Webroot cannot remove these files, as space for newer backups is needed the older backups will be deleted. This will delete the threats from the backup as well.  Another option available to Time Machine users is to exclude the files and folders from being backed up by the Time Machine. You can add them to the exclusion list which will permanently block the files/folders from being backed up in the future. By doing this, the infected file will eventually be deleted from the backup over time and prevent it from ever getting re-introduced to the drive should it be installed on the computer again.
 
Please have a look at the Mac PC User Guide  http://live.webrootanywhere.com/content/553/Changing-Scan-Settings
 
If you have concerns about these files then you can always Submit a Support Ticket and they can confirm if these files are indeed False Positives or not. This is a free service with an active Webroot subscription.
 
Hope this helps?