His argument is that other republican states will use this as precedent for disobeying federal law, I’m just not sure how he hasn’t realized Republicans don’t care about precedent at all

I’ll attempt to suss it out a little bit, as you seem to have given his argument it’s due, even if it doesn’t click for you. It doesn’t for me either. My cynicism tells me that, if he weren’t eying a White House run, he would have just signed it. However, as pretty much the presumptive establishment Dem favorite, he’s been listening to the ghouls in the DNC who say things like “Trans rights are a losing issue, we have to divest ourselves of ‘fringe’ politics”. As a smokescreen for that bit of realpolitik, he is couching his stance in terms of “eroding executive authority”.

My understanding (IANAL in Cali or otherwise) is that California law currently grants the office of the Governor exclusive discretionary power in whether to honor extradition requests from other states (provided the governor is not otherwise compelled to respond in a certain manner e.g. a federal court issues the request or something). Newsome’s argument is that it is improper for him, a term-limited outgoing governor, to “willingly” divest the office of an exclusive discretionary power like that. It’s similar, in principle, to the arguments some years ago regarding the Senate confirmation of political appointees during a President’s second term. I.e. it wasn’t “fair” that a lame duck might have a chance to stuff the government with “their guys”, possibly sabotaging the incoming President of a different party. Not exactly the same, but the principle stands, I think.

Of course, when McConnell made those arguments, he was lying through his teeth, as is Newsome, I assume. It’s just a convenient cover story that makes the common clay of the New West (y’know…morons!) think, “wow, that’s a level-headed so and so with a mind for good governance and bipartisan cooperation, that sure is something that this country could use!”

Where the argument falls down is that, as you pointed out, the Legislature passed this. He already had the perfect smokescreen to say, “there’s nothing I could do, I’m no loonie lefty, but the people of California spoke through their elected representatives and I had no right to veto it merely because it slightly reduced the power of the executive office.” He chose not to, because he’s deathly afraid of the pro-maga-ganda machine painting him as a coastal elite out of touch with middle America, as if that’s not going to be their exact tactic against him regardless of his record. I’d stop just short of him saying he’s eroding the Legislature’s authority though. He’s using the veto power of his office as it’s intended. In theory, overriding the passage of legislation popular enough to make it through that whole process will harm him politically, which is happening to an extent. This is a biased space, obviously, but clearly his decision is unpopular here. Whether or not it helps or hurts him in primaries or the general remains to be seen.

I would hope that the legislature could override his veto, but I am not familiar enough with California state politics to know if they have the votes.