Newsom said he vetoed it because he didn't support legislating an implementation detail -- that all OS/browser makers were required to include support for sending a Do Not Track signal. It sounds like he'd have signed a bill that just said companies were required to respect a Do Not Track signal if one was sent.

> But Newsom said he is opposed to the new bill's mandate on operating systems. "I am concerned, however, about placing a mandate on operating system (OS) developers at this time," the governor wrote. "No major mobile OS incorporates an option for an opt-out signal. By contrast, most Internet browsers either include such an option or, if users choose, they can download a plug-in with the same functionality. To ensure the ongoing usability of mobile devices, it's best if design questions are first addressed by developers, rather than by regulators. For this reason, I cannot sign this bill."

See: https://archive.is/tNlCR