^There's a lot of people in prison, that's for sure. If you look at the overall number of people imprisoned for lengthy sentences for non-violent drug offenses, I guess it seems pretty bad, but it's not really all that clear cut. The US federal drug laws are unfair and very harsh, but only a small percentage of the people arrested for drugs are charged federally. The laws vary dramatically from state to state, and county to county.
To actually get charged federally for growing marijuana would be very "unlucky", but the chances of getting charged federally unless you were part of a group that was being investigated is very slim. The Feds aren't really so much about being out on the streets looking for drugs, although it does happen, but most of their cases are referred to them by local law enforcement agencies. If a state agency stumbles upon a group of people dealing drugs across multiple jurisdictions, there's a good chance the feds would pick it up because they're not limited by geographical boundaries.
I guess what I'm trying to say...There's so much focus on all these people that were handed incredibly harsh sentences for drug offenses in the US, and in a country of over 300 million, that ends up being a lot of people, but it's still a very small percentage of the overall people charged. Most people end up in their local state criminal courts, which depending on the state are generally much less harsh than the Federal criminal courts. It's actually pretty lenient in many places. The chances of being thrown in federal prison for a "personal" amount of drugs or even small time dealing are a lot slimmer than it gets portrayed in the media.
Australia seems no less lenient in reality. On the Australian Border Patrol show that I watch, people are handed out long sentences for large quantities of drugs just as much as in the US! Where in Europe, aside from maybe Portugal can one get caught with tens of thousands of dollars in drugs and get off with a slap on the wrist? Nowhere that I'm aware of...and most of Asia is even harsher than the US and EU.