This map explains it all.  We even have a higher rate than China, which has many human rights issues.

Why is it?  Is it because we give harsher sentences for smaller crimes?  I don't know if that's true - In Japan, smoking a joint can land you two years in the cage.  Is it because we have longer sentences?

I do know one thing.  Personal wealth is a factor.  Wealthier people can more easily buy themselves out of prison sentences as they can higher the better lawyers, etc. 

Locking people up is a business in the U.S. People profit from it. I'm from Jamaica and we don't have lax sentences as the U.S. but people don't profit from another's incarceration either. Too many benefit from someone else being locked up in the U.S. for the prison population to become any smaller.

This. Plus we commit the most crimes.            

http://www.nationmaster.com/graph/cri_tot_cri-crime-total-crimes

because of racial profiling

War on drugs...
http://en.wikipedia.org/wiki/Incarceration_in_the_United_States

In recent decades the U.S. has experienced a surge in its prison population, quadrupling since 1980, partially as a result of mandatory sentencing that came about during the "war on drugs." Violent crime and property crime have declined since the early 1990s.

You have an education system that is set up to protect incompetent teachers an benefit unions at the expense of students ... If you don't have the basic literacy skills and/or numeracy skills to perform well in our economy the likelyhood that you will end up in jail (due to criminal activity) is drastically increased.

I'd say its probably the mentality American's are raised with. This is my right that is my right. Also the culture in the US the biggest stars are criminals. Fifty Cent etc...etc...etc.. I'm not that suprised though its weird the US is as bad as Russia. But either way I think its this whole entitlement shit. Japan for example has a culture of honour and respect as such kids are raised with that respect and such and the older generations treat each other better. But in the US the culture is a culture of crime, doing drugs is awsome, busting a cap in a guys butt is funny, getting shot nine times makes you a hero. I mean seriously the US is a crime driven culture. Also as someone else pointed out all the guns, everyone is packing even old grannies. In the end I just think culturely the US is a much more crime driven country.

As an American I really can't disagree with any of this. Where I am 14 or 15 shootings happened over the weekend and out of those 6 people were killed.

I hope that's a joke.

And my answer to the OP's question is like the Clash said in the song "I'm So Bored With the USA"...because killers in America work 7 days a week. i.e. Americans commit a shit ton of crime. 

Well that and because cops in America seem like dicks from what I've seen on shows like Cops. They arrest people just to look tough and show their authority. People get arrested for having small amounts of pot, underage drinking, etc. They need to focus on major crimes like murder, rape, theft, etc. and less on harmless crimes like pot posession and underage drinking, etc. 

Because we arent screwing around!

America does everything kick ass, including lockups.