I didn’t think promotions are contractually obligated usually. As in you’re not guaranteed a promotion and it’s not written into your contract. So if Amazon, or any other company, wants to change the expectations for a promotion then as long as it is clearly communicated and given time to be adopted I don’t see a problem if they want people to work on site. Especially if working from home is, also, not part of your contract.
You don’t have to work for Amazon if you disagree. Find a, much better, job elsewhere.