Shopping doesn't crawl the web in any way. Retailers submit their stock data feed to it and can then pay to have specific products show up in the embedded box in search terms.
Which is where the anti-trust issue obviously comes in. All retailers can submit their stock feeds to all comparison engines - but only Google Shopping gives you the opportunity to flow through to a pinned spot at the top of the world's largest search engine too.
It doesn't though, does it? It certainly doesn't find the cheapest item on amazon when I tried a few examples.