I'm not in California and DraftKings blocks me connecting with a VPN, so they just don't seem to be distinguishing whether a VPN user is in California (which is the impossible part of this law).
A VPN/proxy could exist on any given IP address at any time, which the California law recognizes but the Utah law does not.
So in theory sites could block all VPN users (if you can somehow "perfectly (1) detect VPN/proxy users"), but if the law necessitates that nationally/globally then it's unconstitutional anyway. It's the age assuring of VPN-users specifically from Utah part that Utah are relying on being possible for the law the be constitutional.