Who decided on these things? You certainly don't. It's game mechanics and class design that dictates what can and should be done, not class/role name.
Game mechanics dictate that when healing is not needed and DPS can be done, the logical thing to do is DPS, because encounter design requires enemy HP bars to be empty. Just because a healer is called a healer doesn't mean DPS isn't part of their job description if they have the tools to DPS.