Starting in March 2010, Smashwords has been buying ISBNs from Bowker (the company who manages the system) on behalf of the authors and publishers who distribute their ebooks through Smashwords.
They got another 50 thousand ISBNs this past week, and these are also free for any ebook that is submitted to the Smashwords Premium Catalog.
An ISBN is basically a serial number for books. It's not an absolute necessity - unless you want to get into a certain ebookstore - Apple. Smashwords started this in March of last year because of iBooks. Apple requires that each ebook in iBooks have its own ISBN. it's a little over the top, but that's Apple for you. It's also rather expensive for the self-published author; a set of 10 ISBNs costs $275 and you can't buy less.
Because of the volume order, Smashwords definitely got them for less than $27.50 a pop, and they probably paid close to $1 each. And that is why they buy the ISBNs in volume.
For more details on Smashwords and ISBNs, click here.