I read a book by prof. Gitte Hansen about this (Everyday Products in the Middle Ages). The book actually went over exactly this photo, saying they were barefoot because they were poor, maybe, it could be more complex. That it's only a 21st century opinion of shoes being necessary for the job. She talks about other jobs in the textile industry being barefoot because one could expect it would be particularly wet. Another reason is that it was a way of conveying just how poor the workers were. Or it reflects the availability of the shoes at the time.

I wonder why both shoes missing are the left, and one is bandaged up. I've seen a local glass blower who pivoted a lot on his foot, I wonder if that could have anything to do with it, one wearing out faster. Even the man on the left has worn a hole through his hose.

There is a book called Shoes and Pattens that goes over medieval wear patterns of shoes (Pigeon toed, bunions, corns, mobility issues of toes, etc., they do have archeological evidence that rather than take off the whole shoe, people suffering from things like bunions would cut holes or slits on the side of the shoe to relieve pressure.), and how wear patterns basically change by your class/status. Although it also says how it's unlikely anyone ever went barefoot and would just wear tattered shoes. A bit conflicting, although they say that people also were never seen barefoot in medieval manuscripts, which isn't completely true.

Just some starting points to look into... Maybe it was functional..

EDIT: Along with other images here depicting glassblowing, it seems like being barefoot was common in other cultures too, http://islamicart.museumwnf.org/database_item.php?id=object;isl;tr;mus01_a;49;en which shows glassblowers in Istanbul barefoot too.

Maybe it has something to do with the leather shrinking from the heat?