I think this change is mainly to encourage people to use repositories (where .debs can be updated later and can grab dependencies) rather than installing random debs. Techies can install .debs using dkpg or red-pill mode if they know what they're doing, so they don't lose out.

Regarding other 'app stores', I assume the web-browser and app-manager will still allow one-click installing through .install files (which add a repository and install a .deb and dependencies in one go).

So overall, I don't think this is Nokia trying to restrict competition for downloading software, just encouraging software to be installed the 'correct' (i.e. via a repository) way.