I was surprised that he ran siege against www.google.co.uk without some kind of "don't do this" disclaimer to new users. Running it against other people's websites is pretty poor form.

Google has rate limiting protection from a single IP. If you make too many requests, they send you to a captcha page.

If you run it against a public site, that may be what happens. However it's intended for stressing your own sites in a test environment, in which case you have full control over the config. Setting up your test environment to block your stress tester as a DoS offender would just be silly.

If it brings down the site, you might get into trouble. It's totally consider DoS.