I personally think it's the latter.

There are estimates that 60-90% of schizophrenics smoke cigarettes, which is 2-3x the normal smoking rate. I think schizophrenics are more likely to smoke marijuana around the time the disease begins to manifest. It's a lot easier to look at your child/brother/uncle/whatever and say "weed did this to him, it's all weed's fault" vs "the disease presented concurrent with a time period where person used cannabis".

I'm not even slightly convinced that weed induced schizophrenia is real.