> Why does anyone have to tell people not to do this? How does it enter anyone's mind as a thing to do in the first place?
Its pretty much the natural naive composition of conditionals. I think eventually most people come to the point where the rightward-marching blocks become irritating and they look for ways to avoid them, and in the case of conditionals where one branch ends in a return there is an easy way to do that, but that particular case (and, thus, the solution) may not be as familiar to people coming from languages where error returns aren't idiomatic (though you do run into it elsewhere, just not as often, so its less likely to result in deep nesting if you don't pay special attention to it.)
Its pretty much the natural naive composition of conditionals. I think eventually most people come to the point where the rightward-marching blocks become irritating and they look for ways to avoid them, and in the case of conditionals where one branch ends in a return there is an easy way to do that, but that particular case (and, thus, the solution) may not be as familiar to people coming from languages where error returns aren't idiomatic (though you do run into it elsewhere, just not as often, so its less likely to result in deep nesting if you don't pay special attention to it.)