My response was maybe a bit much, but you suggested I haven't really used unity because I have never tried to decompile someone else's code, which is ridiculous. I mean I could understand doing it to try to see how someone else implemented some particular feature, but it is still not great to be decompiling and studying other people's code without permission.

I've just never tried to decompile it because I don't make it a habit to steal other people's code. I have seen decompiled XNA code before though and it was a mess. Method and Variable names were not preserved. In either case though when it is open source you actually have permission to take it and use it in your own game. With a game as complicated as this you will also want to be able to use the associated assets as well. Also comments will probably be pretty useful to have.

Decompiled code is really not at all like open sourced code. It is basically unreadable for a human when you have a large project like this. I mean, I guess every XNA game is "open source" as well then.