UPDATED: Purism is using Debian GNU/Linux main to create PureOS. The rest below is for historic purposes.
Purism has decided on Trisquel as its base GNU/Linux distribution, primarily since Trisquel refuses to include nonfree firmware, but Trisquel goes beyond just that, Trisquel also supports a strict philosophy of only including free software as well as promoting other freedom ensuring software.
From Trisquel’s website
There are literally hundreds of GNU/Linux distributions designed to fill every conceivable niche. Only a handful of them are entirely free software; Trisquel is one such distribution. That’s why Trisquel is endorsed by Richard Stallman and the Free Software Foundation.
Does Trisquel include proprietary software?
No, none whatsoever.
Purism is very pleased with the decision of Trisquel as its base GNU/Linux distribution.