Of course it is fair because it is theyre project and nobody is in this game to do charity. They should work for some kind of compensation and beying payed in tokens means that it is in theyr interest to add value to that token. In every bussines the managers board are payed better than the rest of people. The most common percentage i saw that the devs keep is something like 15%. And lets not forget that there are some costs in order to run all the operations.
So yeah, i believe that is only fair and the best way that we know that they give theyre best for the project.
While it is fair, it is a very negative trait and will most likely ruin your project. "Pre-mine" is an instant alarms trigger in the cryptoworld when it comes to scam. Ideally, you want to be able to find other ways to get money yourself out of your project. Typically, no one believes in your project or few people do, so just like satoshi, you should mine your own project right at the beginning of the launch, or whatever other method to quickly obtain tokens but do it after it's released for everyone to see. While some pre-mined coins like Darkcoin succeed, it is a very strong burden to carry in the long term. ZCash for example is ruined because of the stupid coin distribution where devs get a % from.