Just ran into this issue again today when testing the LXD backport.
This is pretty annoying as it's causing a completely different user experience for cloud users compared to regular distro users.
Also, at least in trusty, apt-add-repository doesn't know how to add backports, so a user will need to manually update /etc/apt/sources.list and then run apt-get update which makes installing backports significantly harder than it should be.
Just ran into this issue again today when testing the LXD backport.
This is pretty annoying as it's causing a completely different user experience for cloud users compared to regular distro users.
Also, at least in trusty, apt-add-repository doesn't know how to add backports, so a user will need to manually update /etc/apt/ sources. list and then run apt-get update which makes installing backports significantly harder than it should be.