I'm not so sure it's a straw man, I often see defining constants like this cargo culted even if there are only one or two uses. In that case 'A' is great because it's value is right there, I don't have to look at the assignment and then go look up what the actual value is, so it's more readable.

When it's used in several disparate places then ASCII_A is better and your arguments about correctness should take precedence, we sacrifice some readability but it's worth it.