Becasue if the mood takes any one of them, they'll kill the others. And it seems they're in a community tank too, there are many times of even just one male in a community tank who decided that actually, he didn't like the other fish and killed/bit most of the other fish in there. Bettas aren't livebearers, they'll quite happily attack anything they don't like. Including themselves, sometimes. Oh, and don't take advice from people who work in pet shops, as a general guide, their main aim is to sell, sell sell. Which is why they keep brackish fish in FW, sell goldfish bowls and, indeed, sell bettas to people in who's tank they won't work in.