In most scenarios of medical treatment other than for mental illness, the adult patient has the final say concerning accepting treatment or not accepting it. In the case of a DNR for example (it stands for Do Not Resuscitate) the patient's choice is to pass away naturally instead of facing life as a "vegetable" hooked up to life support machines. In most medical scenarios, the exception to someone having a choice concerning treatment is when the patient is in a coma or in shock and can't be consulted. Even if a person is having a heart attack and needs to be treated urgently, it is my understanding that, if the person is still conscious, physicians will try and get a signature.
In the case of forcing medication on someone with mental illness, the presumption is that the patient has lost the ability to judge, and thus is unable to make a rational decision concerning treatment. It would be nice if a person with mental illness could have more of a choice concerning their treatment options. The problem with us having such a choice is that, when psychotic, a person tends to believe that they don't have a problem and that everyone else is acting crazy.
The existing laws (and the structure of various agencies) are designed as much to protect society from us as they are to protect us from ourselves. Thus, in the case of persons with mental illness, doctors have an agenda other than just the wellness of the patient. Medication is an expedient way for hospitals to get people under control. Trying to explore other options would be extremely expensive, would be time consuming, and probably would be unworkable in general.
Had I been given an option to go off of medication when I was younger, I probably would have done so. If so, I might have become increasingly ill and could have reached a point of no return. I would be living in a vegetative state (but in this case due to untreated psychosis) similar to the one that people avoid when signing a DNR order.
When I attempted to quit medication when younger, I became acutely ill, was hospitalized, and was involuntarily given medication once again. I did not have the opportunity to try to recover without medication in a supervised setting--and this probably wouldn't have worked.
Do I think it's fair? No. I think that in an ideal society, people could be given more of an opportunity to recover without being forced to take these awful drugs. Do I think it's constitutional to force drugs on a person in the name of treating their mental illness? It might not be. So far as I know, there is no clause in the U.S. Constitution that takes mental illness into account. Rights should not be taken away without due process.
Psychiatric drugs can deprive a person of their liberty, their self-directed pursuit of happiness, and sometimes their life (when someone has a fatal reaction to these drugs.) And yet, untreated mental illnesses can deprive a person of the very same things.
When the U.S. Constitution was created, antipsychotic medication, ECT, and psychotherapy did not exist. There was no need at the time to safeguard the rights of persons with mental illness or, on the other hand, to advocate for our treatment, since, at the time, persons with mental illness became the town drunk or the town idiot, or were in shackles at a primitive insane asylum.
I know that medications seem to work for me. It apparently isn't practicable or practical to give a choice to a delusional person concerning being medicated. This is a complex, thorny issue that doesn't have a simple answer. We have an imperfect system of government and medical treatment. Any attempt at a solution to protect society's orderliness and people's perceived safety or to protect persons with mental illness from ourselves is going to be imperfect. Mental illnesses are complex diseases.
To the quandary of involuntary treatment or not, a simple answer isn't appropriate. There is naturally a dilemma to this, and any solution will be unacceptable to someone. The compromise that currently exists, in which persons with mental illness still have some rights, is probably the best that can be done under current social circumstances.