If the information is marked public then crawling it is how the web works, or at least how searching and indexing works.

I ran into that a bit with our startup (http://infostripe.com) when doing demo's it was sometimes shocking to people that with a bit of searching I was able to make a complete profile of their public online activities.

I think that even when people know a particular site is public on it's own they sometimes don't make the connection between software and search engines aggregating all that together without their involvement. Usually this is not a problem for most people but I have seen instances where a user would use the same username on very different services and get burned for it.