I don't see a good reason to view it as akin to a power-grab. It's profoundly unhelpful when terms are diluted and muddied. We should use the OSI's definition not because they say so, and not for legalistic reasons like trademarks, but because it's important to have meaningfully precise language.
It's right that when a thread is submitted of a project falsely claiming to be open source, the HN community unfailingly tears it to shreds. We don't want the term to be cheapened into becoming meaningless. I don't see that the origins of the term are relevant.
Personally I tend to capitalise, I typically use Free and Open Source software to be more clear that I'm referring to free software as defined by the FSF, and open source as defined by the OSI. It's unfortunate that I've found this to be necessary, to close the door on obtuse misuse of these terms.
Ideally they would have picked a term that they could prove to have invented and could have trademarked, then there wouldn't be an issue.
I suppose so, but the term is typically understood in the software community as a pretty precise term of art, and people shouldn't try to muddy it.
It's still better than free software, which invariably makes the uninitiated think of freeware.