Now that I'm at the studio, I just checked a bunch of my 57s. You can turn the grill, but it doesn't just flop around on the top of the mic. They may have just not crimped that one on very well, or that one could be one of those chinese rip-offs.
A 58 is literally the same mike with a ball head. The only difference in sound is caused by the ball head preventing you from getting the capsule as close to the source as you can get a 57. This can be remedied by unscrewing the ball off of the 58 to essentially make a 57 with no grill.