As much as I want to agree with you, there is one possible constitutional issue: the Commerce Clause.
In Gibbons v. Ogden (1824), the Supreme Court ruled that any aspect of trade which might potentially significantly affect interstate commerce falls within Congress' power to regulate. In Wicker v. Filburn (1942), the court, building from the Gibbons ruling, ruled against a farmer growing wheat on his own private property for his own private home consumption, which was never sold on the market, on the grounds that if everyone could grow their own wheat then nobody would buy it, thus significantly affecting interstate commerce. (Because, in that case, the government wanted to price fix the price of wheat.) In Gonzales v. Raich (2005), the court ruled that by and through the power of the commerce clause that Congress has the power to prohibit medical marijuana in all states.
I'm guessing that the argument against Prop 19 would basically follow that the government has an interest in squashing interstate black market trade which would flow from California into the other states. I think our only hope is that by the time the case reaches the Supreme Court that enough justices will be sympathetic with broad popular opinion and with the number of states which will also, by then, have legalized weed, for the justices to pull an amazing opinion out of their ass. Only Clarence Thomas has a consistent track record to protect personal liberties, but maybe there will be a surprise.