> forcing a known wrong assumption by default sounds a bit odd to me
Problem is that in most common cases it's right. The mirroring effect is generally desirable whenever you have a front-facing camera - i.e. almost always on laptops/desktops - and on mobile devices in 50% of cases.
> a simple gsettings key
The "just add another setting" solution is strongly discouraged in the Gnome HIG / ecosystem :) (and indeed kamoso has such a setting).
