I think you are really onto something, although it looks like the main objective was not really to use up some of your bandwidth, but rather to manipulate your site's usage log to create what appear to be extra links to their site. That is, to make search engines think that your site actually contains links to theirs, to help move their site up in Google (etc.) search results for keywords that appear on their site.
A Google search for "referer spam" yields some interesting info, such as:
It looks like part of the problem is if sites around the Internet have their access logs available to be viewed by anyone, this can make them targets for this sort of spam.
The wikipedia article I linked above explains the problem, and outlines some solutions.
