McDonalds is a bit of extreme scenario. Sell it in specialized shop, where everything is explained, where you have to sign paper that you do understand the consequences of trying this drug.
Because how would you stop Mc'D from putting it in your burger for you?
Some people would stop eating there, but most just wouldn't care. Convenience beats ethics for most of the time; otherwise none of us would be buying bottled water, or Adidas shoes, or Apple computers, or any product from any company that was found to abuse people in some way or another.
How do you stop them from putting caffeine in your soda?
Easy, because we have laws about food and drugs. If someone puts caffeine in a drink it must be labeled. If someone puts salt in food then it must be labeled. Do you imagine that government and society would spontaneously decide to take methamphetamine less seriously than caffeine if it were fully legalized?
