2. Amazon can leverage this to guide healthcare providers and employees toward treatments which benefit Amazon rather than the patient. A simple example would be favoring outpatient treatments which allow the employee to keep working rather than inpatient treatments which don't, regardless of which is actually the appropriate treatment. They don't have to explicitly instruct doctors to do this, they merely have to fund outpatient more, make it more prominent in their internal documents, and create more paperwork hoops for people to jump through to get inpatient treatment. Similarly, they can try to steer women away from pregnancy (and therefore maternity leave) by overstating the dangers of pregnancy, and overfunding tests which are likely to create concerns.
3. Medical providers are legally unable to share most medical data with your employer. This becomes effectively unenforceable when your medical provider is your employer.
This information can be used in ways that harms workers, such as terminating employees with costly medical conditions, conditions which might cause them to miss work, mental health issues, or conditions with a social stigma such as AIDS or alcoholism.
Before you say, "they can just use their healthcare through their medical insurance like before for things they don't want their employer to know about", consider that people frequently don't understand privacy, and don't understand that this information could be used to harm them, so many people won't do this. People commonly make the mistake of assuming amoral corporations will behave ethically. Privacy, especially medical privacy, should be default-on.