What kind of markets is this referring to?

Polymarket still charges 0 fees. While Kalshi's fees+spread can approach or even exceed traditional sportsbooks on some markets, neither of them have any interest in banning winning players, as they don't take a side on any bet and directly benefit from more betting activity. Kalshi also pays interest on bets, which can add up on longer term positions.

Betfair has operated with a similar model for decades in the UK and elsewhere.