IdenTrust, who sells SSL certificates as well (among other products), is listed as a "Gold" sponsor for Let's Encrypt:
https://letsencrypt.org/sponsors/
Furthermore, it was announced they provide a cross-signature so that Let's Encrypt certificates will validate automatically for most users:
https://letsencrypt.org/2015/06/16/lets-encrypt-launch-schedule.html
Why did they do this, or rather, what incentive do they have? Is it that they have all those other products as well and may swap their SSL business for the improved brand value?