How does MaxMind prevent someone from releasing an open source version of their database? If you are about to answer "Copyright", remember - you can't Copyright facts. This has been upheld in the Courts system many times.
Facts cannot be copyrighted, but a unique compilation of facts can be. This became settled law many decades ago with sports almanacs.
Furthermore, Maxmind adds non-factual entries to the database so they can identify it uniquely as their work. Map makers do something similar: https://en.wikipedia.org/wiki/Trap_street
MaxMind have two versions of their database, the free to use version called GeoLite and the paid version (whos name I'm not aware of).
I realise this doesn't directly answer your question but I guess one reason might be that they already provide a free version so releasing the paid version would just get you in strife?
Furthermore, Maxmind adds non-factual entries to the database so they can identify it uniquely as their work. Map makers do something similar: https://en.wikipedia.org/wiki/Trap_street