I'm not sure about the ins and outs of maggots and if they can carry disease through their eating...but eating from a manure patty from herbivorous animals would be quite different than eating maggots that have feasted on a dead animal..particularly if that animal is a vector for diseases that may or may not affect chickens. I don't know that there would ever come a time when my chickens needed extra protein that badly that they couldn't go out and pick up some bugs for themselves.