You're the second person who's mentioned being unable to (or advised against) walking barefoot as a result of being diabetic, so I just did a bit of reading. If you already know this, sorry for repeating it, but it seems that the reason for the advice is that diabetes can interfere or destroy nerve pathways to the feet, particularly to the plantar fascia, which can lead to a lack of sensation in that area. As a result, sufferers may be unaware should injury occur, so the general advice is to remain shod at all times.Meh, I'm diabetic and they say I shouldn't go barefooted.
I don't listen when it comes to the beach though, the beaches around where I live are really sandy, hardly any stones at all.
Personally, I'd say that is perhaps slightly over the top - if you can find a surface like a sandy beach or a well-kept lawn where the chance of injuring your feet is tiny, there doesn't seem to me to be any reason not to let your toes see the sun.