Norway rats generally have sweet dispositions as a species. There are exceptions of course, but aggressive rats are pretty rare. That's why they're recommended as a beginner children's pet (feeder or not).
And just so you know, most petstore rats are of feeder rat lines....
No professional rat breeder is going to sell their pedigree rats to a petstore. Just a plain top eared black hooded pedigree rat can fetch $25+ alone. A petstore wouldn't be able to afford that when they're selling rats for ~$10.
And no body wants to breed aggressive feeder rats intentionally. That means potential harm coming to the animals that eat live feeders.