Comment 2 for bug 1849557

The intent of the template files is that they *can* be user editable.
For example:
# head /etc/cloud/templates/sources.list.ubuntu.tmpl
## template:jinja
## Note, this file is written by cloud-init on first boot of an instance
## modifications made here will not survive a re-bundle.
## if you wish to make changes you can:
## a.) add 'apt_preserve_sources_list: true' to /etc/cloud/cloud.cfg
## or do the same in user-data
## b.) add sources in /etc/apt/sources.list.d
## c.) make changes to template file /etc/cloud/templates/sources.list.tmpl

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to

Perhaps the behavior could be changed to be like systemd, with files in /etc/ masking files in /usr .

then the files could be shipped in /usr/ and the user still able to mask them by adding files in /etc.