This old saying feels more relevant than ever in this context:

Mind your own business.

As I see it, there can be various good reasons for striking a "better" deal with some than others, depending on who benefits from who etc. Just like how a retailer wouldn't just pay all the suppliers the same, since they're supplying different amounts of different products that don't all have the same value to the retailer nor customer.

Let's ignore who are the parts in this specific case, but rather discuss the broader principles of free trade. Why would a business have any right to know what their competitors are paying/earning? They can definitely ask as a part of a negotiation process, but in no way can they expect to get an answer. Instead, they can decide not to do business with one who won't share this information with them. This is a good thing.