I think you are making the mistake of assuming that proprietary OSes are never available in source code form. I learned operating systems from MINIX. Nearly al

I think you are making the mistake of assuming that proprietary OSes are never available in source code form. I learned operating systems from MINIX. Nearly all of the source code was published in the back of the text book, and we had the kernel source code in order to modify and experiment with the OS. However, before April 2000 MINIX was neither free software nor open source. I think it's fair to say that this non-open source/proprietary software was not and could not be full of NSA backdoors. Similarly, the original AT&T Unix distributions were also in source code form, even though they were also proprietary. It's very hard (but not impossible!) to hide backdoors in any software when you have the source code and the time to review it in depth.