I agree. I don't even need very permissive license, just to look at sources would be enough. Sometimes I have to lurk around internet corners just to find out simple things which would require few minutes with source code access. It's so frustrating after Java environment where almost everything is available in source code.
The "source" is almost out there. Attaching the debugger to a 64-bit simulator and putting a breakpoint on Apple methods and functions, you can effortlessly read the disassembler code output by lldb/Xcode. I have fixed and learned countless UIKit-related bugs like that.