Well, short answer, because that's the rule the site owner set up when he created the site. But... I've wondered that myself, because @Toe was a big 'blade' fan (which he *also* called 'outside arch'), and many have said they are heel (of the foot) fans also. Some like insteps too. But seems like *most* here want to see totally bare feet with toes or soles in full view. You're probably seen the grief suffered by the arch fans, and those pictures are *allowed* but much-hated by many users.