It's a great point to be aware of, but the title and the tone implies that you as a contributor should be a FOSS maximalist. As they say, not all open source is FOSS. And some people are OK with that. Let them contribute, as long as they are aware of the implications. When I discover bugs in open-source projects during my day job (which is on a closed-source project), I will be much happier having those fixes merged…
He even concludes: "if you are contributing to open source and you want it to stay that way, you should not sign a CLA."
if, if, if.
I generally agree with you (!), BTW, but the author does a great job justifying the when and why of his own opinion, and I find it sound.
IOW, if I want to contribute back because I don't want to maintain a private fork or spearhead a public fork, I just want to be done with it, totally fine, even by the author's opinion, which is fully qualified with the statement "and you want it to stay that way". This is reasonable and not at all forceful like Stallman.
You are reacting to something that isn't written nor implied.