There's usually a lot of turnover at these stores, and cheap fish like damsels don't stay in the tank for long enough to claim a territory. Also, without much in these tanks in the stores, there isn't much territory to claim. And when there's 30 little damsels in a 20 gallon tank, it's so packed the fish can't pick on 1 fish without getting stuck in traffic trying to chase 'em. Most fish which die in the store aren't getting bullied to death, they're dying because of stress from overcrowding, poor conditions, and the stress from being in a plastic bag for a few days.