They aren't, or they shouldn't be, but that's the point of the parent's comment.
Look at the videos flagged by youtube or copyright trolls, a lot of them are not actual copyright violations, but they are flagged anyway by the algorithm and removed or demonetized. And it takes a lot of work to fight those claims.
No, that doesn’t seem to be the point of the parent’s comment. That comment is treating those activities like copyright violations when they’re not. Why would the parent imagine an omniscient copyright bot that is probably wrong?
Me singing happy birthday at home or painting a picture for myself to relax is already demonetized. There doesn’t need to be an omniscient (and wrong) copyright bot to do that.
The first situation isn't copyright violation because some monied entity went out and litigated against Warner/Chappell music. That's the problem with copyright - until you've litigated, which is expensive, you just can't tell what's in and what's out of copyright. You wrote "almost certainly" because of that.
I was hedging against the too-clever HN commenter coming back and saying, “The robot knew you were going to sell the painting” or “You sang the game on the Jumbotron at Yankee Stadium.”
The third exists in a form right now. I've worked for a couple of companies that require all code to be run through a scanner intended to detect if that code has been lifted from open source codebases. You won't get fired if it has, but you will be required to remove it before it is accepted into the codebase.
Look at the videos flagged by youtube or copyright trolls, a lot of them are not actual copyright violations, but they are flagged anyway by the algorithm and removed or demonetized. And it takes a lot of work to fight those claims.