The law does allow you to grow your own. However marijuana can sometimes be
difficult to grow, depending on rainfall and other factors.  The plant can also be
vulnerable to mold.  Also only the female plants produce the desired buds so you
first have to sprout the seedlings and grow them until you can determine which
are the female plants and cull out the male plants. During this process, you may be
in violation of the law as the law limits the number of plants you can cultivate at any 
time.  Then there are those who will steal your plants after all your hard work.
As such many will opt to buy from either these approved clinics or on the black
market.  The downside of black market marijuana, is that you do not know how
they were grown and they may be contaminated with herbicides and pesticides.
You pays your money and takes your chances.