Ask HN: Why would someone want to borrow my site?

I really don't know of a better term to describe what's happening than "borrowing our site", but basically here's the situation:

My company has a private-label version of our site for which we charge a subscription fee. We arrange with our clients that they can direct a domain of their choosing to our site, and we catch the request and template it accordingly.

Recently, I noticed some traffic to our site from a new URL, which has no such account with us. Now, because we don't recognize the URL as a registered client, our site applies the generic DEMO template when this URL is accessed.

I plan on contacting the person with whom this domain is registered, as well as putting in the appropriate filters to prevent this from happening again, but I thought I'd leave it be long enough to ask the question... What does this person have to gain from doing this, assuming it's not just a simple mistake on their part? Is there some security threat I'm missing? All they are doing is sending more traffic to our site from their own domain. Obviously it doesn't help us with search indexing or anything (and it may even hurt as far as duplicate content is concerned), but I wouldn't think it helps them in any way either.

The URL in question by the way is http://travelinafrica.co.uk