We tried licensing the enterprise edition under an open source license but asking people nicely to not distribute it. People didn't distribute it but our customer where very confused about this so we changed the license.
If you don't mind me asking, what was confusing to the customer?
Again, I don't mean to be rude, just curiosity, how would you react if people forked the community edition and implemented some of the enterprise edition features and released it under the MIT expat licence?
The customers didn't understand that they paid to get access to an Enterprise Edition that had an open source license.
If people forked CE to add features of EE we would wonder why we didn't merge those in. For our criteria for merging features that are currently EE only please see https://about.gitlab.com/about/#stewardship
The current license https://gitlab.com/gitlab-org/gitlab-ee/blob/master/LICENSE is proprietary but it does allow people to see the code, modify it and contribute patches https://gitlab.com/gitlab-org/gitlab-ee/merge_requests
Part of the value of a subscription is our support and training, but the unique features of EE https://about.gitlab.com/features/#compare also are important.