the difference between healers doing damage when compared to tanks dpsing or the mentioned bard's songs is because they in some form directly compliment their main role. Healers are not the same. Their damaging attacks in no way directly influence your healing abilities like BRD songs influence their skills, even a Paladin or Warrior's gauge fills up allowing you to improve your tanking skills. Dps skills are just something that was tacked on to give healers something to do during down time (said is optional by the devs themselves) and for solo content.

your comparison is just silly, and compares things that are incomparable.

fyi i'm not against healers dpsing, i am against min/maxers removing the players choice and go by the "my way or the highway" philosophy which is honestly tyrannical.