If you've ever run a web site, you'd be stunned at the amount of garbage traffic you get. Search robots (yahoo, google, bing, etc., etc.), scraping robots (pulling your links, email addresses, and such), nefarious robots (looking for login holes, trying to inject nasty code), etc. It's constant. Thousands of hits a minute on a reasonably popular site. Lots of ways to reject it, but the second you do, they figure out other ways to get in. Never ending battle.