Part of the problem here is that there is too much emphasis on trying to change game and job design to accommodate people who fail at the balance between DPSing and healing that they've dumbed down the DPS aspect and people haven't stopped making those mistakes
But the problem is actually quite simple, yet unresolvable for a very simple reason. People make mistakes. They are human. I like to think I have a good priority over healing, but I am capable of misjudging and if that happens, somebody dies, I also own up to it and apologize.
I'd be surprised if it is because that healer would rather DPS (if they did, they'd play a DPS), chances are, yes, they are trying to play optimally, which is also what your tanks and DPS might be doing, before 5.0 that'd be your tanks dropping tank stance, for both your tank and DPS that might be eating an AoE, but in those cases, it's all right, because your healer has your back. For example, some people will purposefully eat AoEs to keep their DPS up, I rescued a MNK out of an AoE yesterday only for them to run straight back into it (they weren't even in a position where they'd lose stacks)
Tanks make mistakes. DPS make mistakes. And healers will also make mistakes. Let's not berate them for it and instead help them learn from them and stop screwing with job or game design.
It is inefficient for a healer to keep everybody's health 100% of the time. We are playing a game where our performance is marked by rDPS and healing is also marked by out overheal too. And many people will strive to improve on that, because the end result is they are a more efficient healer and makes their contributions greater.
But you can misjudge a tankbuster, you can misjudge their gear, you can misjudge how well the tank uses their cooldowns. I mean, i've healed the same pulls over and over and found tanks whose health slowly drops, so I can do more DPS, I also see tanks whose health drop very quickly. If I am used to the former because I have mainly got the former and not the latter, then there is a greater chance I can misjudge it. I still prioritise healing, but you might think you have your healing in line. Even at 60% health the tank will be fine in exception of a tank buster, if your healer misjudged that tank buster, there's your problem.
And when you make a mistake you learn and do better next time.
But thing is, I am surprised for you to find it is THAT common, because my experience is the opposite, it is rarer. Though if you are finding it is common, it proves my points about dumbing down DPS on a healer not preventing it (and to give us a more complex rotation again)