Because that "personal decision" has a huge negative influence on a lot of people. Drug use affects your family and friends negatively, it affects your coworkers, your employer, random people that you run over with your car or attack violently while high, and it raises taxes or insurance premiums since it's very costly to treat drug-related issues at hospitals.
Sure, in the beginning drug use may be a personal decision that doesn't affect too many people - but once addiction kicks in - and it does to a very large percentage of users - your drug use has a hugely negative effect on everyone around you, and even strangers.