You've just cited a bunch of companies that are separate entities owned by Amazon. US courts have a very strong presumption against attributing the acts, economic or otherwise, of a corporation to its owners.
It would be virtually impossible for California to win in court on the grounds of A9, A2Z, Alexa, or Amazon AWS having offices in California. None of those companies are involved in Amazon.com's separate retail business.
Most individual states would disagree, especially when the wholly-owned subsidiary exists primarily to avoid taxes.
If Amazon's technique was legally unassailable, they wouldn't be paying sales tax even in the 5 states where they already do. They'd just assign whatever creates 'nexus' in those states to new 'non-selling' subsidiaries.
Piercing the corporate veil becomes easier the more closely integrated the companies and their management structure are.
Amazon can maintain arms-length relations with most of their subsidiaries because those subsidiaries are not part of Amazon's core business, and can operate semi-independently with little to no loss in efficiency.
Maintaining such a relationship with subsidiaries directly involved in the day-to-day operations of Amazon's retail business (maintaining stock, setting prices, making sales, packing and shipping product, etc.) would be much more difficult, costly, and risky.
Company A buys software from company B to carry on its business. Company B does not become liable for the acts of Company A.
A9 and A2Z are providing software and/or services to Amazon.com which Amazon.com uses in its retail business. This is completely different from A9 and A2Z engaging in that business. They do not buy, sell, distribute, or ship the products Amazon.com does.
So long as Amazon.com maintains arms-length dealings with A9 and A2Z (which is trivially easy for any company with semi-competent legal counsel to accomplish), the ownership of A9 and A2Z is irrelevant to Amazon.com's dealings with California.
Not really, no. As far as I can tell, they all provide products or services in some for to end users outside Amazon.com's retail business.
Not that it matters. The principles are well-enshrined in the common law system, codified and uncodified. You may think it's ridiculous, but the courts take it seriously.
It would be virtually impossible for California to win in court on the grounds of A9, A2Z, Alexa, or Amazon AWS having offices in California. None of those companies are involved in Amazon.com's separate retail business.