It's not rent seeking, which should be clear because the premium domains have a fixed price schedule that is far lower than what domain scalpers would charge, instead of a variable charge based on what they think they could get away with. Even at $850/yr, you can probably renew premium domains for multiple human lifespans before you end up paying as much as the scalpers might ask for certain domains. Instead of rent seeking, it is priced such that scalpers can't afford to squat on thousands of common word/phrase domains.