Eh... I work in a corporate environment, and I think the only part that makes sense is it's a dumb idea. (Not yours the idea of taking the pdfs off.)
More then likely it isn't lack of sales altogether, but instead:
Lack of sales of the official pdfs + an increase in pirating.
It's pobably part of soemthing that's been tracked since day 1. Maybe someone's quarterly report finally showed the pirate to official pdf ratio was in the cancel now range...
What it doesn't account for is that it's new, so obviously more people will be pirating it. (especialy if it is popular.)