Most webmasters welcome webbot from search engines such as Google and Bing. Contents of their websites will be indexed by search engines and users can easily find thier websites. However, they surely not welcome your web spider to extract data from their sites, may be you're up to no good, such as how products from etsy.com were used to drive traffic to a Hong Kong website. Most likely your IP will be blocked if webmasters detect unknown web spider aggressively crawling their websites. Way back to 2001, eBay filed legal action against Bidder's Edge, an auction scraping site, for “deep linking” into its listings and bombarding its service; Craigslist has throttle mechanism to prevent web crawlers from overwhelming the site with requests;
Even big spiders like Google has mechanism to prevent others from scraping their content. Try to search a keyword and at the results page, click page 1, then page 2, page 3,...At page 20 (in my case), Google will stop displaying search results and want to find out are you a human or webbot that reading the page. If you unable enter correct captcha, then your IP will be blocked eventually.
