Decompiling Unity

The ability to decompile / convert managed IL code into any other .NET language is a feature of the CIL itself. Even Visual studio does partially decompile referenced assemblies. MattRix even uploaded the code on a github repo with the permission of Aras. Keep in mind that the code is still copyrighted by UnityTechnologies. Decompiling is ok as long as you don't copy things and claim it's yours ^^. If you plan to extract code from there to use it in your own projects you really should contact the Unity support for getting permissions.

This might not be a good advice but as long as you keep things for yourself, nobody can blame you. However whenever you release something you should be clear about your sources and licenses.

I've also just found this question which still doesn't have an answer (so i'm going to write one ^^). I will tell him the same thing. If you want to use something from those assemblies you really should contact the support.

ps: I have ILSpy running almost all the time in the background (thinking about to adding it to the autostart :P).