Think about it.
Really? I believe you need to learn about the differences of Open Source and Open STANDARDS. They do not have anything to do with one another. For example, TCP/IP is an open standard. Linux Kernel is Open Source.
See the difference yet?
Maybe some more examples:
Open Office = Open Source XML = Open Standards SMTP = Open Standards Apache = Open Source HTTP = Open Standards
Now to use your own logic...it doesn't make sense for closed source vendors to go their own way if they want to play on the Internet. So they leverage Open Standards. And then some enhance the standards when the standards are weak or not evolved enough to give the user a good experience (but if they're smart they make it downward compatible).
Microsoft users HTTP, TCP/IP, SMTP because they are good, mature, reliable open standards.