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.
Interestingly, Google Shopping (when it was Froogle) used to be quite good - back when any eCommerce site could just send in the feed of their stock.
At some point they moved towards Google Checkout (Google Merchant/Wallet?) verification and the number of stores you could actually find with it dropped dramatically.
Now it's just high street retailers, a handful of the bigger online places and eBay. Not only is it not as useful, it's killed off the sites that came before it as they tried to adapt and survive.
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.