While that may be true, it no one should expect to be backdoored by using a proprietary product.
If you don't know how to determine if you have clean water (e.g. the parent comment), then either you demand proof or expect, uh, dynsentery and a bad time. You should not assume you have clean water.
https://www.nytimes.com/2017/05/04/us/flint-water-home-forec...
Vendors with names people trust should be held accountable when they fuck up like this.
Also you can't trust everything all the way down. A Pi still has proprietary video chips, every Intel and AMD board has some kind of management software, etc. etc. There is no way for one human being to be all Tony Stark and build literally everything from the ground up.
People go to the shop down the block, pick a product from the shelf and they are done with it.
This doesn't make them deserving of backdoors.
"Bought the wrong brand of car ... you had that accident coming"
"Visited the wrong doctor ... you had it coming"
"Ate food at the wrong restaurant ... had it coming"
Not everyone is going to be an expert in every field. This is not how a modern society works.
That said, for Hardware this does not yet work, and firmware is of course another problem.
Lets hope for a better future.
https://www.gnu.org/proprietary/ has a list of other kinds of malware found in proprietary software sorted by type and company or type of product. It's very informative reading and touches on issues which often come up on sites like Hacker News.