Its not the masks per se that concern me, it's that they give a very false sense of protection and therefore encourage people to venture into enclosed spaces such as shops, hairdressers etc thinking they are in some way protecting themselves.
There is conflicting evidence on the microbial barrier afforded by the typical blue examination masks. In fact a mask that has been worn for some time harbours quite a soup of potential pathogens. 20 minutes is the maximum usable life. I'd be pretty confident those people wandering around Carrefour don't change masks every 20 minutes. But they will be fiddling with them just to reload their hands with bugs and transfer any picked up bugs to their face.
So there is every chance they do as much harm as good. But at least it makes people think something's being done.