On the other side is an assumption that a publisher can just set significantly sifferent prices in different store fronts and are entitled to being on those store fronts. That isn't the case either, brick and mortar stores have always had the option to not carry an item when the sale price is significantly different than other store fronts.

The 30% thing is also incredibly misrepresented. Regular and online stores always have a significant markup for the vast majority of their stock, with a few high profile items as exceptions to that rule.