Only 8.4% of US prisoners are in privately owned prisons:

http://en.wikipedia.org/wiki/Private_prison#Development_3

I suspect wider cultural reasons rather than just simple corruption - though that no doubt plays a part.

Private prisons are only part of the revenue generated from the war on drugs. Civil forfeiture is a huge income stream for local and federal. They basically wait for the drug dealers to sell their drugs and then catch them with the money and take it. Some police departments allot for 35% or more of their annual budget to come from civil forfeiture. Add to that the amount of federal money received to fight the war. Also fines and fees. There is a lot of money changing hands and jobs being created by prohibition.

It's not just private prisons, it's the correctional officer unions too. They benefit greatly from large prison populations.

I found this:

http://en.wikipedia.org/wiki/California_Correctional_Peace_O...

So they lobby to increase prison terms and to "enforce current drug laws". I suppose they should have the right to do that, but it still seems a bit odd to me!

There are about 1/2 million correctional officers in the US:

http://www.bls.gov/ooh/protective-service/correctional-offic...

So they might be able to have quite some impact by lobbying and voting as a block, but they are not a big enough body to swing votes against a meaningful majority.

(your link states that union has ~31,000 members, California had about 17.5 million registered voters in the Nov 2014 election)

Yes, they do have a right to do that. NAMBLA also has that same right. I also have the right to call them both a cancer to society.

>enforce current drug laws

That reads to me like it could mean "outlaw medical marijuana."

That can be a very misleading number.

8.4% owned, how many are not privately owned have services/goods/staff provided in large part by private companies?

There's always someone who makes this comment, and I'll say the same thing I said before: https://news.ycombinator.com/item?id=8562814

Which is still greater than the ideal of 0%.