Addiction can lead to a number of mental illnesses. For people susceptible to such mental diseases, drug use is very dangerous. It can exacerbate latent conditions, and make existing ones worse. Marijuana, or any drug, should not be used without the advice of a physician. Drugs effect everyone in different ways, and it's impossible to know how someone will react.
There are valid arguments from scientists who say that marijuana is perfectly normal and does not cause any health disorders or illnesses. To fully test this theory is to ask the people who have been smoking or taking marijuana in one form or another. There are many people who speak against it from personal experience and who are indeed suffering from Schizophrenia, Psychosis, or Other Mental Disorders.
The studies only show correlation and do not prove causation.
The studies in question showed a correlation with schizophrenia and smoking marijuana at earlier ages.
However another study showed the same correlation with tobacco. BUT in that case the researchers postulated that dopamine dysfunction could lead to an addictive personality that is more likely to do things like choose to smoke tobacco at younger ages and that this correlation was because dopamine is also related to schizophrenia.
Of course it's also possible that tobacco contributes to the development of schizophrenia, but that wasn't the conclusion favored by the researchers. In fact some research suggests that tobacco is favored by schizophrenics (schizophrenics smoke tobacco at a much higher proportion than the rest of the population) because it helps alleviate symptoms. That's another possible reason for the correlation given a schizophrenic would feel disturbed long before symptoms of schizophrenia manifest. That's also a possible reason that schizophrenics may smoke marijuana at a younger age.
It has also been suggested in some studies that CBD which is a chemical in marijuana has antipsychotic properties. Depending on the strain marijuana may have more THC or more CBD (which actually reduces some of the effects of the THC, though not entirely). Users often refer to the high with a high THC:CBD ratio as a "mind high" and one where it has a higher amount of CBD as a "body high". It's possible then that whether marijuana helps or hurts schizophrenia depends on the specific strain.
In the end we need to remember researchers are human beings. Oftentimes a study will have data that could mean a number of different things but the researchers will for what ever reason and often without realizing it tout certain conclusions and ignore others. All we can conclude for now is that more studies should be conducted.