You can include MIT & BSD libraries in a GPL-licensed app but you can't include the resulting GPL work back into an e.g. BSD-licensed app. Same as you can include jQuery in a proprietary app which has its own license.
They may be compatible but you cannot remove someone else's copyright and license from their source code. They specifically say you must include the license.
Wix may still be in the wrong, but WordPress is definitely in the wrong for removing copyright and license. IANAL but both of these companies would look awful funny in court because disregard for other people's licenses doesn't help your case when someone disregards your license.
As far as I can see the files that were originally under MIT still have their original copyright statements intact, as comments above illustrate. Which files are you alleging have had their credits removed from?
In this case the WordPress-Editor-iOS retained the copyright notice in the file copies from the ZSSRichTextEditor project. But did not include the LICENSE file.
Also the wordpress-mobile developers mixed their source code with the MIT licensed one. Just one LICENSE file I can find in the WordPress-Editor-iOS.
The MIT/BSD projects are not separated out. I think they can claim GPL license on their created code, but that has to been clearly identified. Easier if they removed the MIT/BSD components, and make them git submodules.
In addition, the wordpress-mobile developers created code do not contain a copyright notice.
> The MIT/BSD projects are not separated out. I think they can claim GPL license on their created code, but that has to been clearly identified. Easier if they removed the MIT/BSD components, and make them git submodules.
Using MIT licensed projects doesn't require you to keep the code for the parts you've adopted eternally isolated from your own. If that were a requirement, that would entail an inability to make any improvements without also distributing those improvements under the MIT license―which is directly antithetical to the permissiveness that the MIT license aims for. The parts do not have to be easily separable.[1]
To be clear: mixing MIT source into a project (whether by copying files, or by copying snippets) and distributing the result under a license that is not MIT is something that is absolutely kosher. The only requirements are that you have to reproduce the MIT license text and any corresponding copyright notice somewhere when you start distributing the thing to other people. Look into what Google and Apple do with Android and iOS or type about:license into Firefox for three really great examples of how to satisfy the requirements of third-party projects when incorporating others' code into a project with a license that differs from the original.
The only thing Automattic is in the wrong with here is to have linked to the licenses and copyright notices of the original project rather than include a copy of if inside their own distribution, which is arguably a faux pas in some circumstances and in some circles, but it's easy enough to remedy and no court would ever award past damages for it.
1. For a license that does work like that, i.e., a weak, file-based copyleft, you can look at the MPL. http://mozilla.org/MPL/
