Why does Software Centre mark any local .deb as proprietary

Ubuntu's Software Center (actually Gnome-Software) likes to register itself as the tool to open a local .deb file.

But it always seem to report that app as being proprietary. Even if the file was downloaded from an official repo. Try opening any deb in your apt cache (/var/cache/apt/archive/*) for example.

Is this a decision to try and discourage users using deb files not displayed by the Software Center?

I have demonstrated this to my satisfaction on U1810. I normally use Mate and Synaptic so don't know how long this has been going on for.