Just a guess, but the Svalbards are located north of the Arctic Circle, and weather conditions in the Arctic for a large part of the year, especially high winds, might make deploying bear spray tricky, and deterrence at a distance (the rubber bullets) more advisable if the situation allows.
The other thing to consider is that grizzly attacks are hardly ever predatory, while polar bear attacks pretty much always are. There are no "surprise encounters" in the flat vegetation-free Arctic environment. I'm a big bear spray advocate, but on a polar bear making a beeline for me, that would NOT be my preferred choice.
A 2013 self-defense shoot from Svalbard, from Norwegian media: