If they're spinning, the thread is stripped in the case. Several of the cheaper cases I've used over the years have done the same. You can still use the standoff to hold the motherboard securely, but if you ever want to remove the screw you'll need to hold the back of the standoff with a pair of pliers. Looks like it'd be easy on that case - just remove the side panel, no hard drives or other stuff to remove.
