420 FMX, the government bases most of its assertions on studies and research. Some of that research is outdated, and some is biased, but there are studies and research to back up much of what they claim. There just hasn't been any very large-scale, unbiased testing because testing itself isn't really allowed.

You can read for days on this site and find all the studies and links you want to that stand behind many of the claims. You'll also find some good information that refutes some of the older claims:
Schaffer Library of Drug Policy

You can do your own research into the history of cannabis illegality through Google. It has a factual basis. Just not a wholly solid one.