The real reason they dont want marijuana made legal is because in the 1930's industrial hemp was deliberately defined as a drug. (read Hemp Lifeline to the Future)

Around this time the Duponts, (whose industry was based on oil) wanted to destroy the industrial hemp industry in the US which was going strong for more than 125 years.

And Wm Randolph Hearst demonized marijuana in his newspapers by racist slurs against blacks and mexicans who smoked the "evil" weed.

By demonizing marijuana and deliberately confusing it with non smokable industrial hemp (minute traces of THC) the industrial hemp industry was destroyed when drug laws against marijuana were implimented.

However the ban against industrial hemp was lifted during WW ii because fiber was needed for parachutes and was unavailable when the Japanese took over the nations that grew jute and other fiber. Farmers in the US could buy a certificate to grow hemp which was revoked after the war.

So, if medical marijuana was made legal, it would remove the foundation and justification for outlawing the growing and processing of industrial hemp in the US which would create thousands of jobs, save trees (hemp can be made into paper and even lumber and beams!)

Its all about protecting the timber and oil based industries that now make products from nylon instead of fiber.