"But it's pretty much impossible to square the CLA's requirement that contributors grant Canonical the right to ship under a proprietary license with a commitment to free software. Instead you end up with a situation that looks awfully like Canonical wanting to squash competition by making it impossible for anyone else to sell modified versions of Canonical's software in the same market."

GPLv3 is our default license (unless LGPLv3 makes more sense), both for projects with a CLA requirement and those without. Mir uses GPLv3 because we like the GPL for the same reasons you do.