They're employees. The employer pays them for their time and can demand that they attend whatever activities the employer chooses, from work training sessions to anti-harassment training sessions to sessions talking about corporate culture to sessions expressing the employer's views on topics like unionization.
If you want, you can hire someone to listen to you talk 8 hours a day on whatever topic you choose--there's no law against it.
You have a fair point, but also consider the large imbalance of power here.
On one side you have a mega-corporation that can take away the employees ability to eat, live indoors, or receive required healthcare. On the other you have Joe Six-pack who could possibly mildly inconvenience his employer, at worst.
Granting the mega-corp the ability to speak it's mind, without also guaranteeing Joe a few rights in return is just moving that threat from explicit to implicit.
This is wrong. Your employer cannot ask you to engage in things that compromise your safety or otherwise illegally put you in harms way.
The thing at question here, is whether the implicit threat of job loss constitutes such a case. I would argue it absolutely does - especially for those at the bottom of the income ladder (like amazon workers). This is an existential threat to coerce a behavior.
If you want, you can hire someone to listen to you talk 8 hours a day on whatever topic you choose--there's no law against it.