These folks very well understand that bad people can steal keys. Stolen keys have been a thing since antiquity. It's not rocket science they don't understand. Every single key in history has had a risk of theft and this is no exception.
The truth is a lot of them understand this just like you. But unlike you, they are willing to accept this risk. You aren't. That's fine. You have a value disagreement -- you don't think the risk is worth it, and they think it is. Which is why you make arguments about the merits of the issue, instead of attacking the other side on their basic literacy. Just because you disagree on values that doesn't mean the other side is stupid or illiterate about the basics.
This isn't about politeness either, it's actively counterproductive to your own cause: insulting people and showing that you lack a basic understanding of their position is a sure-fire way to make sure they dig their heels in and continue to oppose you regardless of the merits of the situation. i.e. it's a grest strategy for losing in the end.
On another level, pushing a compromised update to every phone in Europe (to gain access equivalent to having a copy of the backdoor) would be noticed by the publisher, and the binary difference would quickly be found by security researchers.
They're not concerned about their own security. They believe that they will have access to tools that will make them safe, but that "those people" will be deterred, and it is acceptable to give up some basic safety for many to catch "the bad people."
And, historically, they weren't necessarily wrong. The problem is more that all of the tradeoffs and probabilities from history have been thrown at the window, because it is now trivially easy to phish the majority of a population and getting a master key gives you access to hundreds of millions of individuals that you can reach from around the globe rather than a couple dozen that you can get to in person from a particular apartment complex.
Understanding encryption is not the problem. Understanding scale is the problem.
Technical nit:
I (mis?)remember reading in a cryptography book (by Schneier?) that the NSA has at least one algorithm where the decryption key is different than the encryption key, but my search-fu finds no references to it online. The application would be for devices in 'hostile territory' where there was a risk of capture of the unit and so you didn't want the other side to be able to alter data/firmware since the cipher that is available is decrypt only.
However, see perhaps:
* https://crypto.stanford.edu/~dabo/abstracts/traitors.html
* https://crypto.stackexchange.com/questions/39397/one-encrypt...
Also, see n-of-m algorithms: