So it sounds like the tax base percentage is being applied to money earned outside of the state of California which seems pretty crazy to me.
Not really...
Let's say you're in the NFL. We'll say that counting mini camps / OTAs, training camps, meetings / practices / games during the season, required media days, etc, that you're officially "working" 150 days/year. Those are "duty days".
Now assume you're Darnold and your salary is $30M/year, just to make the math easy. That calculates that you're earning $200K/day ($30M/150) because for the purposes of the taxes, only "duty days" count.
Because the Super Bowl players were in CA for 8 days, and because of the high number of mandatory media and other events in addition to practices / meetings / the game, we'll assume that all 8 days qualify as "duty days". That means the tax base for CA is $200K * 8, or $1.6M.
They're assuming that money is "earned" in CA. Because duty days are where your salary is earned.
It's NOT just days spent in CA. If you came to CA a day earlier or stayed 2 weeks after the Super Bowl (as perhaps Darnold might do b/c he's probably got family here), those are not duty days and you don't have to pay anything. Because you're not working.
That's my understanding.