Over 10 years ago we identified that some customers were paying for no remote agent licenses at all but were backing up 100s of remote systems and expecting us to support the environment - at that point our licensing was changed so that ANY remote system has to have an appropriate license even if you do not install our agent into the system concerned - as such a backup over a Windows share DOES require a license.
If you want to see proof of this - then as long as you are beyond the 60 day trailware period, try backing up such shares with no Windows Agent or Application Agent licenses installed on your server at all - if you do this you will receive a license error and the backup will fail.
Note: The recommended licensing package for larger environments is the Capacity Edition which does provide ability to backup any number of remote systems and does not need separate agent licenses