Tanks are here to take DMG and use their GCDs in order to take less damage. Healers on the other hand have a few DPS skills but their job is to keep the party alive. As long they do that there's no reason to demand more. If the healer feels safe enough that there's no threat of wiping then they should DPS, after all it's boring to just sit there and do nothing if no healing is needed.
Everyone asks where did healers go when it was them that made most ppl not feels comfortable with this role and changed to sth else.