The important thing here is that this is about a quasi particle, something that behaves sort of like a particle but is not (like a hole in the electron distribution), in a 2D crystal lattice. This only happens because the lattice is not isotropic, you see a different pattern depending in the direction you look, so having properties change with direction is not totally unexpected. We already have materials with anisotropic thermal conductivity for example.

This won't happen in vacuum as vacuum is isotropic.