The standoffs' threads are machined flat on one side so the back plate can stop them from turning. If you managed to pull the standoff out like this, I'm guessing the backplate or the spacer's threads got messed up from grossly over-tightening.
There isn't much you can do about it. If you really want to remove that standoff, use pliers to hold it while you use a screwdriver to unscrew the screw. Be aware that if you cross-threaded the screw, you may not be able to put it back together: the screw and standoff threads may be too messed up.