Google would say they aren't preferencing their product. They are preferencing the merchants who actually sell the products over middlemen who try to extract rents from said merchants. Because they put free links to merchants' product pages in search results.

TBH it's the same as Google not linking to other search engines' result pages in its search results. Why would it, when it can link directly the content the user wants?

Google is a middle man extracting rents from the merchants. I'm pretty sure merchants that pay for google ads get preferential treatment in google shopping results. Is it any worse than PriceRunner? Probably not, although I haven't used PriceRunner so I don't know.