Here are examples of proprietary software that has something worse than a back door.
The NSA has put back doors into nonfree encryption software. We don't know which ones they are, but we can be sure they include some widely used systems. This reinforces the point that you can never trust the security of nonfree software.
Oracle's nonfree Java plug-in for browsers sneakily installs other annoying proprietary software.
That article disregards all other bad things about proprietary software. For instance, it regards the inclusion of proprietary Flash Player (which has a surveillance feature and DRM) in Chrome as a good thing. Chrome is a proprietary browser with a universal back door.
We don't agree with the article's views on those issues, but we present it as a factual reference.