Here in the coldest darkest wettest parts of Europe (where it seems that spring ain't ever gonna arrive this year) you get this kind of result...
...from road salt and not much use (whether the calipers are working correctly or not) particularly on rear discs that aren't used to the same degree as the front brakes.
Once the discs are as badly corroded as this you need to change them.
Before ripping into a caliper I'd check to make sure if it works.