Again what you are saying is true, but where do you draw the line? Most people consider a DUI a minor crime however the average person who gets arrested for DUI has driven drunk 80 to 90 times for every time they get arrested. Approximately 10,000 people a year are killed by drunk drivers. Regarding crimes such as possession, prostitution, public drunkenness etc. Yes they are minor, but many murders, rapist, and pedophiles have arrests for those crimes as well. Do we wait until someone commits something egregious? With that being said I pretty much do agree with what you are saying, I just can't fathom a way to fix it in our society.
Regarding the vast majority of people in jail not being murders, rapists, and pedophiles it actually depends on the jail. In big counties with fairly high crime rates such as LA, Cook, Wayne etc. most offenders of minor crime do not see much jail time, there is no room, so they get light sentences such as probation. So in a sense many minor crimes don't really receive a big punishment.