Actually SM57's aren't uncommon for vocals. For example, Anthony Kiedis has always done all his vocals through a 57. My guess about using the 57 for Maynard is that the output from the megaphone is probably pretty loud to get that distorted blaring sound. Since 57's are dynamic mics they can handle a lot hotter signal without blowing the diaphragm. It also provides a little natural compression since the transients on a 57 are going to be less pronounced than with a condenser or a ribbon mic. That could help the distorted vocals sit a little better in the mix without having to process them much.