Every stoner has a theory why marijuana is illegal. But its usually inaccurate. The reason marijuana is illegal is because the united states was filled with religious nuts 100 years ago who were only ok with alcohol. We then forced prohibition on most of the world...
OP: yes, you can make BHO out of fan leaves and stems, but it's going to be some weak BHO. You are going to need a lot of shake/stems to make it worth your time.
Reading about the history (especially only ~80 years ago) of cannabis is not very difficult guys. We actually have several threads about it in this forum...
Of course our government keeps it illegal because of the $$$ flow to prisons (25% of inmates incarcerated in AmeriKKKa due to drug crime, inmates = incredibly cheap labor / slaves), LEO, etc. Plus hemp is a viable alternative to blah blah blah
Hell, do some reading about CIA / Pakistani ISI involvment in the opium trade. "Conspiracy" 101.