It's actually meant to say if someone provides an exception, e.g. "No parking on Wednesdays", then that proves the existence of another rule, e.g. "Parking is allowed". Since an exception, without a rule, makes no sense.
But, in my experience, people do use it to mean "Oh, this one thing is wrong, but that proves everything else is right", which does not track.
Give it a try if you don't believe me. Even categories we take for granted, like trees and fish, are not perfectly crisp, and "obvious" facts like "humans need a heart to live" have surprising exceptions.
> Pointing out that penguins can't fly doesn't make the case that birds can fly stronger in any way.
I disagree. It's such a common rule that there's a long Wikipedia page for the exceptions[1], and the first photo is of penguins, labelled "penguins are a well-known example of flightless birds.".
If I knew nothing else about the topic, I would take it as evidence that it's common for birds to fly, otherwise that fact would have been unremarkable. Not hard proof of a universal quantifier, but a useful rule nonetheless.
Yes, this is why hard and fast rules don't make sense, and why they should have "generally", "normally", or "mostly" attached to them.
If you have two categories of birds, one with those that fly and one that doesn't, having that second list doesn't make the first stronger. At some point that second list dilutes that first one so much that it doesn't make sense anymore.
If my rule is that "white guys are named Dave" does my building a list of every example of a Dave and non-Dave make my rule stronger? When does the "strong" nature of the rule get watered down sufficiently? Honestly, a list of hundreds of birds tells me that it's a weak rule and that the "birds fly" rule is wrong.