Neither move is surprising, since both are dealing with significant injuries and are only depth players even when healthy. Cumpton had Tommy John surgery and then shoulder surgery, so he won't be a factor next year. Sadler just had Tommy John surgery and likely won't pitch next year either.
The two pitchers' injuries and outrights must be frustrating for them. Both have been decent in limited duty in the big leagues (or, well, Sadler really just had that one very good start last year after struggling in 2014, but that one good start is surely fresh in his mind). Even before they got hurt, they had to fight the perception (a correct one, but still) that they're best suited to be backup plans. Now, they'll both will have a very long road back to the big leagues, if they ever do return.
The moves open two spots on the Pirates' 40-man roster.