Actually the desktop helpers are looking for them in $SNAP/data-dir
This is because the mount fails when the directory already exists. For an example see:
https://bazaar.launchpad.net/~ubuntu-desktop/quadrapassel/snap/view/head:/snapcraft.yaml#L26
Ultimately we don’t want everyone to put this into their snaps, we want developers to use the upcoming template feature in snapcraft to prevent lots of boiler plate yaml that ends up being hard to maintain.