This may be theoretically true, but it is not practically the case. There is a reason Linux and OS X users almost never use antivirus software.
Using Windows on a medical device is inexcusable. It's a heart monitor, not a game system.
Today, doctors from a hospital the other side of the country can diagnose your cancer realtime while you are still in the MRI machine, and they do it with Windows, because that's what humans know and use.
Probably because they've been exposed to it in a many settings, for better or for worse. Windows being popular does not imply that Windows is appropriate for any given task.
> Why shouldn't the devices use Windows?
Is this a serious question?
Windows is probably the least stable production OS in the world today.
Windows is extremely bloated compared to an appropriate embedded OS.
Windows is (practically speaking) the only OS where antivirus software is a fact of life. Viruses should not be a concern for medical equipment.
Windows is not even close to real-time.
I don't want to see "Your heart monitor is restarting for updates in 3... 2... 1..."
> and they do it with Windows
This may be technically true, but this in no way implies that Windows is uniquely suited for or appropriate for the task. They could also do it using a PlayStation, but this is probably not an appropriate platform.