IIRC GitLab is "open core" which just means it's not fully open source and Mattermost is not really fully open source either, it its own confusing way.
The purpose of open source was to give users full and unrestricted power. To free them from lock in and allow them to be the masters of the software they use.
It seems that these companies view open source as a marketing trick and perhaps a way to save on development costs. They're making a mockery of open source and should probably be shunned for it.