If you could get meth at Mc'Ds, would you take it? No? Then why do you presume that everyone else would?
Congratulations, you are now addicted to meth.
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.
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?