That is dictated by where consumers end up shopping. You can diversify to thousands of stores, but if Amazon is the biggest seller, they will get the lion's share of the click throughs and purchases.
What PZ did was show users who shop at other stores, that Amazon was cheaper, and funnel users to Amazon. Sometimes Amazon wasn't cheaper, but I wonder if this is all about to change.
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I'm the owner of PZ. I spoke to the associates team for technical workarounds, including not using the API at all (e.g; purchasing the data from a 3rd party), in order to retain associate's status. The manager said it wouldn't matter if we abided by the ToS or not, Amazon did not want PZ.