Amazon told 3rd party sellers it wouldn't use their sales data for its own sales (and possibly contractually agreed not to? - unsure here), and it now seems that they broke this promise.
How other stores operate doesn't matter. Amazon said it wouldn't do this and used that trust to attract sellers and grow their platform.
Which is an excellent reason for preventing these types of monopolies from forming in the first place. Once you let one retailer get so large, they have almost unlimited power to act in this way, but the people who they harm have very little power in receiving any recompense for it.
> The consequences range
They sure do.. and Amazon is one of the largest companies operating today. The range is "off-scale high."
Stealing your customers sales metrics, yeah that's never okay.