As a frequent writer on these boards, I'll admit that the concept has crossed my mind when I create stories. For me, the justification is usually the unwilling transformee deserved the change or ends up enjoying their changed form more. But like BrotherOriginal said, this is fiction. We're allowed to stretch the boundaries common decency. The 18+ is a constraint placed on us by the real world, to avoid any of the stigma/prosecution of child porn. However it also makes sense in a worldbuilding way. You should be a legal adult before deciding on something as life altering as a transformation (granted, that's only for voluntary transformations).
That being said, some involuntary transformations do have a negative impact on a victim's life. Some of these can be akin to a major accident (such as Hetoki's Ride of a lifetime), where the new transformees have learn to deal with their new existence. The result would be similar it they had simply lost limbs or ended up paralyzed. Another example would be the B-virus series, which features both animalistic and gender changes. While cruel, there's no malice behind the changes. It mirrors the spread of a real life disease, but the effects are rather benign compare to say Ebola.
And finally, you're right. There are some involuntary changes done that I would call "rapey". I think the Cruelty of Man contains several scenes like this. However, if you don't like something that extreme, you don't have to read it. The genre is always going to have it's more extreme end, both in context and the transformations themselves. It's not something that can be fought, but you can try to act as a voice toward more moderate/friendly transformations by providing support (emotional, proofreading, financial, etc) to those authors that more closely alight with what you want to read.