It's not about getting someone healthy, it's about getting them to a point that they are of sound enough mind that they can make rational decisions.
If you hear voices that tell you that antipsychotic medication is going to kill you, you won't take it. The idea that you're choosing to not take medication of your free will is wrong - your free will is compromised by a mental illness so severe that your perception of reality isn't even correct. How can you possibly decide what's best for yourself if you can't accurately understand the world around you?
The point of this kind of forced treatment is to get people into a state where their brain isn't lying to them and they have a basic understanding of what's happening, so that they can then make decisions.
The folks who say it's unethical to force this kind of treatment sound like they're sticking up for folks with severe mental illness, but they're really just condemning them to suffer from that mental illness for life, because without some kind of outside help they simply can't take the actions needed to stop suffering from it, because it prevents them from doing so.