Do you think it's the case that consuming marijuana will cause someone to become schyzophrenic when they never would have done so otherwise?
Or is it the case that it may act as a catalyst, and cause the outbreak of the disease to come sooner rather than later.
In any case, I don't think this has anything to do with the issue of legalization:
First, I believe that the debate is only about allowing canabbis use for adults, and even the people that support legalization think it (like alcohol and tobacco) should not be legal for minors.
Second, if the history of smoking tells us anything, is that even when there's proven (and not just speculative) damage, and even when this damage is not just for a tiny segment of the population but essentially to most people who smoke, and even when the damage is not just to the user but also to those surrounding him/her (including small and unborn kids), still it's not considered sufficient reason to make the substance illegal.