Actually, I disagree.
It makes perfect sense. Not that I am one of these people but say it's a sunny day and you go indoors, you can't keep your glasses over your eyes because you can't see, you can't put the glasses in your pocket or they might break or snap, putting them on the table (if you're sitting down) only takes up space if you're eating a meal...or you could walk out the establishment without the glasses forgetting you had laid them on the table. You could however, put them in your jacket but you generally wouldn't have a jacket with it being summer. so....looking at it like that it is pretty practical just to rest them on your head.