I broadly agree with this being an unfortunate outcome but you do understand that making candidates who failed your interview want to work at your company is fundamentally limited in how much it actually helps you. Yes, yes, I know some of them may come back and pass the next time, or they tell their friends about how you were super nice and gave them great feedback, but this is pretty rare. If you're doing this, you're doing it out of the goodness of your heart, not because it helps your recruiting pipeline. And, even though I agree with the idea of providing feedback, assuming that people will have positive feelings when you tell them why you didn't accept them is misguided. I have friends who I know personally that have gotten interview feedback and not taken it well. Of course I tell them to shut up and stop poisoning the well for everyone else, but the point is that this is largely not the picture you are presenting it as.

How do you "accidentally" end up saying something that implicates you in discrimination on the basis of legally protected characteristics - what are some examples of that?

This has always felt like an excuse used by people who who just don't want to be caught in their own lies when asked to come up with a real, non-discriminatory reason.