Site referral - why is it done?

A web site with 690,000 "hijacked" pages

[ammonet]

I found a site that has around 690,000 pages indexed in Google, all of them referrals to other sites, ranging from the White House to my feeble little sites. I'm not even sure of the technical term for this kind of proxy activity. The URL takes this form:

http://www.example.org/bp.php?url=http%3A%2F%2Fwww.anothersite.com

I suppose this will cause a duplicate content penalty for thousands of sites. However, two questions:
1) What positive gain does the perpetrator obtain? PR? Traffic?
2) What can be done about it, if anything? I know where the server is housed, but is what they're doing "illegal"?

[edited by: tedster at 6:06 pm (utc) on Mar. 19, 2006]
[edit reason] use example.org [/edit]

[Rosalind]

This is probably a 302 hijack attempt. It's legal, but unethical when done deliberately. A search for "302 hijack" will give you plenty of threads about this on WebmasterWorld.