If one in a million healing mages (who are already relatively rare) is capable, would you say that they're capable because healing mages are capable, or would you say that because they're part of that specific squad of freakish monsters they are capable, regardless of being a healing mage? Generally, the latter, not the former. Sure, being a healing mage that can internalize their own magic is part of what makes Usato monstrous... but the rest of the Rescue Squad can't do that (It was a big thing that only the Captain and Usato can do it), and they're still all freaks, too.