Nothing. It's the same as nvidia's non-gpl kernel modules. The simple fact is they will never be accepted upstream, but that matters little to distributors of ubuntu's scale.
In the case of Nvidia's modules, Nvidia's proprietary licensing disallows distribution of a prebuilt nvidia.ko (as that implies distributing a modified version). Coincidentally, their license terms for the OpenSolaris driver have no such restriction and the OpenSolaris descendants distribute the prebuilt module without potentially violating Nvidia's license terms.
Amazingly, their Linux licensing used to be worse. They used to claim you were only permitted to install the driver on one computer within an organization
Someone hired better lawyers. All these ridiculous EULA and click through licences and idiotic mandatory registration systems we see, I can't help think many companies would benefit from hiring better lawyers. Get rid of the timid who default to 'no' in order to protect their own arse, hire people who help you get where you want to go.
In this case, I can't even see any real liability issues - even if Canonical did get taken to court there are no damages since the software is free of charge.
People decided to listen to lawyers who read the licenses instead of listening to statements by people claiming to know how things work without actually reading either license or asking a lawyer about it. That is quite literally the only change.
No, I've seen talks by the engineers behind Solaris (I don't recall who at the moment) that strongly indicated the Sun lawyers didn't go out of their way to be incompatible with the GPL -- they just wanted a license that allowed them to split proprietary and open code (as they didn't have the right to open up all of Solaris due to licensing agreements with third parties etc) -- and still being able to distribute both open and traditional closed Solaris. This led to the "per file" license nature of the CDDL -- and unfortunately to the "additional limitation"-bit that makes it incompatible with the GPL.
If it was done again today, they might have gone for the Apache license as I recall -- and avoided some of the unfortunate issues.
Pretty sure Canonical saw an opportunity with container management, plus interest in ZFS, decided to get over the unfortunate licensing issue and support the module.