Server-side checks cost processing power and memory hence they need to spend more on servers.

Client side kernel-level anti-cheat only ever consumes resources and cause problems to the actual gamers, not directly to Rockstart's bottom line (and if it makes the game comms slightly slower on the client side it might even reduce server resource consumption).

If Rockstar's management theory is that gamers will endure just about any level of shit and keep on giving them money (a posture which, so far, has proven correct for just about every large game maker doing that kind of shit) then they will logically conclude that their bottom line won't even suffer indirectly from making life harder for their existing clients whilst it will most definitelly suffer if they have more server costs due to implementing server side checks for cheating.