What are the advantages of doing this rather than just using or forking AOSP?
The tl;dr is that Amazon already uses a fork of Android called FireOS. However, Google makes manufacturers sign a contract that says that if they manufacture devices that run Android, they cannot manufacture other devices that use forks of Android. This was making it difficult for Amazon to find manufacturers that could make device with FireOS, so Amazon is ditching Android entirely.
0: https://arstechnica.com/gadgets/2023/11/amazon-fire-tablets-...
This would prevent Amazon from shipping those but they wouldn't be refused by factories.
This is more likely about control/NIH.
I mean, it's almost literally what MS was doing with windows back in the 90s, isn't it?
If it's an anti-trust violation, then it's an anti-trust violation whether it's a fork of Android or not. If it isn't an anti-trust violation, then Google can ban things that aren't forks of Android too.
If it's legal for Google to force manufacturers not to use Android-based FireOS, then they could also force manufacturers not to use Vega-based FireOS. It feels like Amazon needs to challenge the legitimacy of those Google contracts or else they may find themselves in another situation where Google just amends their manufacturer contract to disallow Android forks and Vega-based FireOS.
Actually, the law often turns more on the intent of the person being investigated than on what they do. (Is this a good idea? No.)
Google can claim a more convincing fig leaf when they're regulating Android than they can when they're just saying "also, you can't work with Amazon". And that slight difference in plausibility can make a big difference in whether the same contractual clause is legal or illegal.
"Hey, y'all can have our source code, but no one big try to copy our business plan with it"