Those "prongs" are supposed to "dig" into a wood door for added stability. Obviously they weren't designed for metal doors but if they are"grinded off" the lock will work for your metal door.

I would return it, and buy one that doesnt have them, before I would mess around grinding them off.

I just now saw your reply, cat_ky. I had already used a cutoff wheel to remove them. No problems with that, and all is good. I had considered returning it, but decided against it because then I wouldn't have been able to get one with a shape and finish that matched the other door in the room. Oddly enough, the regular "passage" and "privacy" knobs in the very same line don't have these spikey things.