These better work conditions would come at a cost. The money has to come from somewhere. You can't just complain that Amazon doesn't treat their employees well enough without explaining where you would get the money to improve their work conditions.
Sounds like a good reason to publicly shame amazon, thus damaging their brand value. If the damage to their brand is high enough, decent working conditions pay of. People start to be proud to claim that they "stopped to use Amazon".
Or what about some tighter regulation of their working conditions? Sounds uncomfortable - might be cheaper to just improve the situation of the workers ...
The unions are just doing their work with Amazon, and they are doing it very well.
If society decides that even low-paid workers should be treated with a modicum of respect (e.g. actually getting paid for the time they work, not being treated like automatons), it's the employer's problem to figure out how to pay for work conditions that meet the standard.
From their bottom line. Maybe the prices they charge are too low. If they can't afford to do business by the law, they shouldn't be in business. Someone who actually complies with the law will fill the void.
You can't just complain that Amazon doesn't treat their employees well enough without explaining where you would get the money to improve their work conditions.
You can't just complain that homeopathy doesn't cure cancer well enough without explaining how you would cure cancer.
So why not create a charity, with everyone paying 10 cents (or more!) and donate it to Amazon workers? Or to anyone else who has entered into a voluntary agreement, who someone else thinks is wronged?
Do we shed a tear for the coal-fired plant operator now forced to pay for scrubbers? No. As such, we shan't cry over Amazon's costs rising ever so slightly to internalize the previously externalized cost of marginalizing their warehouse employees.
When an employer doesn't pay an employee a fair wage, they're being subsidized, either by the government (Walmart is an excellent example; their literature specifically instructs their associates on how to collect government benefits due to them not paying a living wage) or by society as a whole.
From an economics perspective, would you not prefer consumers be responsible for the full cost of the good/services they're receiving?