No offense, but that wasn't well-communicated in your original post. Partly because...
...it's not normal, and in my experience, not common either. I grant my dataset may be skewed because like 70% of the time I am the healer, but I really don't see this often. And certainly nowhere near as often as your post gives the impression you think it happens; saying it's so prevalent gives the strong impression that you are under the impression that a healer doing damage at all is giving up the opportunity to heal.
I mean, I would say that it's also not normal for a tank to refuse to enable tank stance and not AoE mobs, just focusing on single-targeting a single mob. I've seen that, though. I've actually seen that more than I've seen DPS-only healers... though admittedly less than I've seen Cure-spam healers who will just do /emotes or stand there when there's no healing to be done.
But I wouldn't say any of those scenarios are common, certainly not enough to post like it's some sort of epidemic of people acting that way in-game. "I've run into a few healers who will DPS to the exclusion of healing, and I'm finding that frustrating" is a statement I think many people can agree with. "80% of mages are toxic glare spammers, and we must stop this epidemic!!!1!!11" comes across a bit differently.
However, if I had to guess, I'd say that people are suddenly feeling sensitive to it because of Bozja. (A theory supported by the various times Bozja or Critical Engagements have been explicitly mentioned in this thread.) It's still early days, people are still trickling in, and many folks have not learned the mechanics for critical engagements yet. This is a recipe for Much Death.
Simultaneously, it's a combat setup we're not used to. Critical engagements feel like organized duties; they have complex mechanics and they're off a separate circle where other people can't come in and join. But in trials, raids, and alliance raids, you have organized parties; if I'm in Puppets' Bunker and the healer in another alliance goes down, I can notice it in their alliance window, select that healer, and rez them. (Assuming I've got Swiftcast, and some other healer doesn't try to rez them at the same moment.)
But in a critical engagement, I can see only whoever I happen to be partied with; I might have ended up solo-queuing and not be in a party at all. And that gets rough. I see someone dead on the ground over there, but everyone's running around to avoid the angry red horsebird and I can't easily click on them to rez them, etc. Someone else is injured, but trying to click onto them to target them for healing can be difficult in the crowd; I can't use function keys or the controller selectors, because we're not in a party or alliance. I do try my best to keep the main tank healed up if I see them hurting (because I can target the boss and hit 'T' to swap to who they're targeting), and if someone does a '/say Rez pls?' or something, I can raise them because I can now target them from the chatbox... but otherwise, people in Bozjan Critical Engagements are probably on their own when it comes to assistance from people outside their party.
And that's unfortunate. If you end up on the ground in a Critical Engagement and sit there, seeing an AST or WHM still attacking the boss while you're on the ground, I guarantee it feels frustrating. (Hey, I've been there; it is frustrating.) And so it probably makes "healers who DPS rather than healing/rezzing" feel like it's suddenly a more common thing, and I suspect it makes people sensitive to the perception of it elsewhere in the game.
Maybe you're in the Bunker and see one healer who legitimately is clearly DPS'ing rather than healing your party, causing them to die; yeah, that sucks, but normally you could just brush it off as one terrible healer. Then you go into Bozja and in two critical engagements you see people on the ground while a healer is still upright and DPS'ing. Then in another Bunker run your party is down, and you see a healer in another party doing damage. But hey, you also had that Ridorana run where a healer did come through in the clutch and rezzed your party's healer, thus saving everyone! And being sensitized from the one experience you might well go, "Four out of five times, healers just left us on the ground! 80% of healers are toxic selfish DPS-only jerks!"
But the truth is likely that in Bozja, trying to get up other parties in a sane manner during a critical engagement is often a lost cause; you just have to opportunistically rez where you can and hope you get the right/useful person up. And in that second Bunker run, that healer might've had to use Swiftcast to rez someone else you didn't see go down, or had to use Swiftcast to open up a spot to move themselves out of the bad, or whatever else; slow-rezzing someone in many of those fights is a Bad Time waiting to happen, so they might well be biding time until Swiftcast came back up.
The healer in that first Bunker? Yeah, they were probably just a bad healer; that is a thing that happens.
But even with this ludicrously insufficient hypothetical sample size, that's only 20% of the healers hypothetical-you encountered, not 80%.
Are there some bad healers out there who will focus on DPS rather than healing? I don't run into them often, but yeah, they do exist. And even good healers can have a bad/distracted day where they're off their game and get tunnel vision on "We can burn this boss down quickly, I know it!" I don't think anyone in the thread would argue with that.
But saying that those bad healers are 80% of all healers in the game and it's an epidemic of toxicity? That's a bit of a wild claim, and the sort of thing that people—especially career healer mains—are going to push back against. And with a claim of it being that widespread, you kind of have to expect that people are going to read your complaint as being about healers doing damage at all.