The difference is that when, for example, DPS just spam one skill and their DPS tanks, they don't turn around and try to argue that that's acceptable play. They know it's bad and just do it anyway because they're lazy. (Some) healers, on the other hand, will actively argue that only healing and standing around otherwise is "good". That's a misconception that deserves to be addressed separately and given special attention. That's why Healer DPS gets more discussion than other sub-par dps. It's a matter of clearing up a false concept of "good", as opposed to just pointing out that they're bad.