Btrfs snapshot as user is not permitted

My backup user should be able to create a btrfs snapshot of the system subvolume (mounted at /). But this doesent work.

Setup: subvol=/rootfs is mounted on / and subvol=/ is mounted on /btrfs

First:

then as backup user:

If I change the owner of / to be the backup user, it works, but is this the right way?

Ubuntu 16.04 / Linux 4.4.0-59-generic / btrfs-progs v4.4