There was an early iPad video on Apple's site that showed the lego icon while browsing a website (which they later edited), so it was there at least initially.

Apparently Apple eliminated the icons to make it less obvious that their browsers weren't displaying all of a site's content.

Someone has hacked together a way for sites to override that behavior: http://www.gskinner.com/blog/archives/2010/04/return_of_the_...