All L1 visas are internal transfers; they require extensive documentation of the role being performed outside the US for an extended period, and tie you to a specific set of responsibilities in the US; and they are not transferable once in the US - you can’t move to an L1 at another company, because you haven’t worked for that company outside the US. THe difference between a and b is just the kinds of roles that are eligible for transfer.
If you are on an L1 and you quit or are dismissed, you lose your immigration status and have to leave the US.
Since it’s much easier to get hired and potentially get sponsorship from within the US, So I’m guessing this is why Amazon added the non-compete.
Green card application takes forever and is never certain. It will take years, all while you have to stay with the same employer (unlike with H1B visa). And if you are an Indian national, then it goes from "multiple years" to "closer to a decade or longer".
As for "through sponsorship", this has nothing to do with L1. "Sponsorship" means finding an employer that would be willing to file for an H1B visa for you. Whether you have an existing L1 visa or not has no bearing on that process. It is no easier for another employer to file for H1B visa if you already have an L1 visa. Finding a company willing to sponsor your H1B is the exact same process and difficulty with L1 visa as it is without one.
Overall if you come as an executive or managerial staff or are promoted to one within your employment you have a fairly easy green card to attain that H1B and many other visa holders don’t.
As for the rest don’t underestimate the value of local networking and recruitment.
https://www.upcounsel.com/non-compete-california
I remember Apple and Google getting into trouble over non-compete several years back.
Plus we're really talking about processes on different scales. Individuals moving elsewhere doesn't say much about the pace of creating companies to pursue novel innovations.
I'll also note that CA not having non-competes makes it easy for people to leave and start companies elsewhere. The real question is whether those new companies will create dynamic local scenes of their own, in the same way the Fairchildren did here. If they don't, and if Silicon Valley keeps being a hub of innovation, that's a great sign that banning noncompetes still matters.
1: https://en.wikipedia.org/wiki/San_Francisco_housing_shortage
Adding some context: as I understood, L1B is a "transfer" visa for "existing" employees, which under "existing" often mean record of employment being outright forged, or being put on very low key position with explicit intent of later L1B transfer. L1B is supposed to be of lesser value than L1A, which opens much more options for an employee/corporate serf to away.
I believe the change is due to the later being a much easier door to US for a lot of people from low income countries where Amazon runs captive outsourcing centres. They didn't want people to scout for another, better jobs around the moment they stepped into US, even if that mean the next hiring company had to do its own L1 for them.