The reasons which you mention
Yet this is a question of both labor law, labor rights and citizen rights. In my eyes this makes it a political question. Corporations over many decades have been shown to be deeply entangled in the "political processes" of government with, for example, an army of lobbyists in Washington.
Another example of the matrix of entanglement, is the Citizen's United case which gave private corporations free speech rights.
deal with how Amazon acts as a legal entity in the United States. Which is separate from how it acts as an employer. As every employer, Amazon has a right to impose any restrictions which are not illegal to impose.
The workers have a right to refuse and voluntarily employment, which (while it can be punitive by its nature) is not considered a punishment because it's not coerced under duress. It's considered a reaction to the existing market conditions. And while you can argue that the market conditions create a form of duress, it's not the same as the primary beneficiary of this action creating duress.
If Amazon locked people in rooms and didn't let them out (physically restricting their movement) until they agreed to certain terms, that would be different. But there is no indication of anything like that, or anything even remote like that, happening.
Which makes this a question about actions of employers with respect to employees. Since the employer is not a government, this is off topic on this site.
If you want to know more about how employers can restrict employees actions, you may ask a question about it on law.SE.