One thing is a bug, another one is a backdoor. Are these good faith bugs or willful backdoors? Most likely they're bugs, but it is hard to know.
If I was Microsoft and I wanted to willfully plant a backdoor, I would take precautions to be able to get away with it if caught. Because security researchers can analyze them, and foreign governments have Windows source code, leaving intentional bugs as the only choice.
Now, the reasons I am suspicious of Microsoft:
- PRISM. Which is unequivocally mass surveillance.
- The Flame malware was able to install itself via Windows Update: http://www.computerworld.com/article/2503916/malware-vulnera... . The means by which the Flame authors achieved this are easier to explain if they received help from Microsoft.
- When Windows NT SP5 was released, the build accidentally came with debugging symbols (i.e: variable names were visible in binaries). A researcher found a variable called "_NSAKEY" containing a key which could be used to forge signatures. https://en.wikipedia.org/wiki/NSAKEY. Microsoft's explanation was that it wasn't related to the NSA, and that NSA in that context meant something else.