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My guess is that they either have their own bot doing it or are paying someone to do it for them. Apparently (although I've not noticed it), this is a new form of viral marketing some companies are resorting to.
For example, a referrer of:
is almost certainly logfile spamming. Not easy to detect if you just own the one domain; but when you own nearly 50 and see the exact same referrer to all your domains within a relatively short period it stands out a mile.
Just add them to your mental list of companies to never do business with.
adds: the DMOZ data dump seems to be a common source of domains to go logfile spamming on...