It makes no sense, it is the person's money and life, and it is theirs to ruin as they wish. We are not properties of the state. If a person cannot be allowed to do what they wish with their own money, because they might harm themselves or others as a result, then how can you trust them with driving a vehicle, flying a plane, operating weapons in the military, or even owning a personal weapon?
Every gun sold to a person is a gamble on whether they use it to cause harm on others (same with the things i listed above).
This same logic applies to regulation of drugs in general as well in my opinion. Regulating other peoples lives is not the purpose of the government, especially when they're not harming others or being a nuisance to the public.
Crime goes up, bankruptcy goes up, corruption in sports goes up, etc.
I agree that people should be given freedoms, but we live in societies and people aren't independent, disconnected, autonomous units.