Sun and Moon seems to be branching off from the Pokémon fromula quite a bit (replacing Gyms with Trials, possibly eliminating HMs in favour of Poké Ride, etc.) and it appears that they are also adding alternate forms of Pokémon based on location. Do not forget, also, the illustrious and magical 35-foot-tall Alola Exeggutor, gaining the Dragon-typing from a mixture of its sauropod appearance and the Dracaena Draco. Making Vulpix and Sandshrew Ice and Ninetales and Sandslash Ice/Fairy and Ice/Steel, respectively, is just one of the new directions that Sun and Moon appears to be taking the Pokémon franchise.
Also, have a look at the southeastern island on this map of Alola:
As you can see, there is a high mountain covered in ice. While not confirmed, this is most likely the place where Icepix and Iceshrew will be found.
Hawaii does have snow. This is the peak of its highest mountain, the dormant volcano Mauna Kea.