According to the tag's wiki, exposed_breasts is used for when "Breasts are visible when the character is wearing clothes that would normally conceal them." However, a great deal of pictures have been added under this tag, where characters just simply appear nude or among such things, wich doesnt match the description of the tag.
My guess is it's because maybe on other sites "exposed_breasts" is the same as "breasts", instead of being different tags and having different meanings here on e6. So then people use the tags they're used to from those other places here on this site, and this happens.
I have a lack of personal judgement so I dont think its right to do everything myself (nor the time to go tag hunting every post, really), but I think it's a good idea to bring this to attention.