The Democrat decided to reject the measure because it applies only to the biggest and most expensive AI models and doesn’t take into account whether they are deployed in high-risk situations, he said in his veto message.
Smaller models sometimes handle critical decision-making involving sensitive data, such as electrical grids and medical records, while bigger models at times handle low-risk activities such as customer service.
Newsom's press release and veto message include much more detail and suggest "it's too weak" is not the actual reason.
Reasons mentioned:
Suggests Newsom is going to be very hostile to any legislation that is designed to deal with X-risk concerns, and that he, frankly thinks they are bullshit. (I personally am also pretty skeptical of X-risk from AI, but I don't want nothing done given how bad the risk would be if it did manifest.)