I highly doubt the US consumes 80% of all "illegal" drugs in the world. Where does that number come from?
The US does have highest incarceration rate in the world, which is mainly due to locking people up for possession of small amounts of drugs.
Imprisoning people for such small offences leads to a downward spiral of drug use and criminality, as you don't learn how to be a decent citizen in jail.
Decriminalisation of small amounts of drugs has a positive effect on the society as a whole because people are gonna use drugs anyway and imprisoning them for such small offences only leads to more problems.