Alternative (Alt) Text is meant to convey the “why” of the image as it relates to the content of a document or webpage. It is read aloud to users by screen reader software, and it is indexed by search engines. [Harvard link]

Oh, I see what you mean! So strictly speaking, alt-text should be used for the intended purpose, i.e. in order to help certain users and searches, etc.

Fortunately or unfortunately, I suspect most webcomics makers, and no doubt others, are going to keep using it the way they are. I'm not sure there's a real issue in this case, but maybe in others..