Historical Album Sales Data - Why So Hard?

While reading Weird Al's Wikipedia article, I noticed it seemed a bit out of date on the album sales, listing 12 million total albums sold, as of 2007.

So I took a moment to see if I could locate a more recent sales count.

Via this site, I found Nielsen SoundScan, a subscription only service.

So, no closer to my goal, although admittedly this would provide the bleeding edge total of album sales. From a Weird Al forum post, from 2005, it cost $600 at that time, per album, to see the sales data.

Four worldwide organizations seem to currently certify sales figures.