I bet someone technically stole it from a ISP since they likely were suppose to return it. Hard to say it really depends on the actual seller since it appears that site appears to allow third party sellers. This is like some of the stolen cell phone people buy off amazon from time to time. The only reason it is not out of hand is amazon will force a refund and kick the seller off the site if the seller does not fix the problem.
Those firmware release do not make the router any cheaper to buy in builk in fact the ISP many times has to pay extra to get the special firmware. So there would be no reason for a third party vendor to not sell the standard ones since unless they were buying used equipment and so called refurbishing...