I wonder how many people contributed under the assumption that their contributions would remain under GPL. I assume the CLA specifies their right to change the license?
I'm not a lawyer, but it's my understanding that the existing licenses still apply to the existing code but new versions can always be licensed under a new license (on each release if they wanted), because of the very fact that it's just that: a new release.
The important point is that they have a CLA and that one contains copyright assignment. The company behind LightTable holds all rights on your contributions. As the copyright holder, they can change the license however they wish.
"Good" is a definition that differs, depending of the side you are on.
Also, what is "non-free"? Some FOSS advocates would say that they just did that, because it now allows building non-free derivatives for everyone without user-rights to the source.
The point is that the free software movement views things from the end-users perspective: the user is entitled to see and use the source of every binary blob they get delivered and use. That's an important point of the GPL. FOSS is all about freedom for users.
MIT allows the _developer_, who is not necessary the user, to modify the source, build a binary blob and deliver that to users without ever letting them see the source. The freedom from the users perspective is 0, except those that the developer gives them in their terms of service. MIT is all about freedom for developers.
Which one is the "free" your CLA aims for?
The ambiguity problem with using the term "free" is a well-known issue.