Backup doesn't usually back up a program's files. Since Programs need to be "Installed", it makes no sense to simply back up the program's "program files" folder if no User data is stored in such folders. Since that folder would be fully rebuilt upon it's reinstallation, why does a backup need to be made of it?
I tested this also, and while a backup file was created, the zip folder was empty as was yours.
But again, backing up such a folder makes no sense and is a waste of space in the backup file, so it only makes sense that certain file types are excluded from being backed up.