And the proof for that is? In the pudding. Do you really think that psycotic people are rational enough to stand trial? Do you think that they just take the drug then pretend to be rational afterwards? No. The fact is as I stated, they are not rational and fit for a trial unless the are properly treated.
You don't understand how "anti-psychotics" work. Much if not most of their "therapeutic effect" lies in their ability to clobber the brain. Patients do not as much become rational, as zombies. In the old Soviet Union, publically advocating democracy and or free market reforms was not infrequently diagnosed as "schizophrenia." Haldol and the like were equally effective in putting and end to these "delusions." Critics of the regime describe how, just as their economic delusions disappeared, even their ability to remember their kids' names went too.
The truely scary thing is that if one's belief that one is innocent is dxed as "psychotic," it's quite likely that haldol and the like would be able to get innocent people railroaded.