Drug companies have done that for years. It's called Marinol. It's synthetic THC, prescribed to people for the effects of chemo and as an appetite stimulant for people with AIDS. it's been approved for use since 1992. They can patent Marinol and restrict its use, but they can't patent THC, any more than Coors can patent the formula for alcohol.
Why do you draw some sort of distinction between the use of a substance for pleasure and the use of a substance for its health benefits? The two aren't mutually exclusive. I like the taste of an apple, which you could certainly describe as pleasure, and an apple is also beneficial to me.
Marijuana laws are a lot like gun laws; an overreaction to a perceived threat. Marijuana laws have their foundation in racial bias against blacks. It was perceived as a "black drug", only present in the black community, and that it was dangerous because it could incite black people to commits acts of violence against whites. Much of the credit for marijuana legislation can be laid at the feet of William Randolph Hearst, and propaganda in his newspapers in the 1920's. It was categorized as a dangerous narcotic based simply on hysteria, long before anyone did any serious scientific studies on its effects, and even now, the remnants of that hysterical propaganda keep it on the schedule for the Controlled Substance Act. In fact, Marijuana continues to be listed as a Schedule 1 Controlled Substance, in spite of years of actual research, which classifies it as even more dangerous than Heroin. That's absurd, but it remains there because of years of propaganda.
The long term effects of marijauna, and the effects on society as a whole, are orders of magnitude less than the effects currently caused by alcohol abuse. Marijuana laws remain in effect for one main reason: that it's not socially acceptable to smoke weed because of the stigma of years of propaganda, but it's socially acceptable to get bombed on bourbon.