Well, I guess you can't believe everything you hear.
The 7% number is from Wikipedia, but is only about Federal Prisons, which only has something like 220k prisoners compared to State prisons which are, as I posted, 1.35M. Even looking at the actual numbers I don't see 7%:
http://www.bop.gov/news/quick.jsp
102,580 50.6% Drugs
30,756 15.2% Weapons, Explosives, Arson
24,311 12.0% Immigration
10,480 5.2% Extortion, Fraud, Bribery
9,697 4.8% Sex offenses
8,403 4.1% Robbery
7,161 3.5% Buglary, Larceny, other property
5,563 2.7% Homicide, Aggravated Assault, etc.
I guess it all depends on your way of looking at things, but I would say that the Drugs, Immigration, Extortion and burglary categories are the ones that aren't necessarily violent crimes; the rest adds up to way more than 7%. Immigration is a mixed bag: it's a growing category in part because it's easier to get a conviction; you arrest someone under suspicion of some other crime, then find out they are undocumented and file those charges instead. How many of them are violent criminals? No way to know. Is it 0%? Unlikely.
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OIC, here's the source:
http://www.drugwarfacts.org/cms/Prisons_and_Jails
I see they don't count sex crimes as violent. Nice.
Anyway, there are three times as many violent criminals in the State prisons than there are in the *entire* Federal prison system. The Federal system, while at historic highs, is a relatively small piece of the picture (15%?).
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There are entirely too many people in jail and prison in the US, but there's no need to wildly exaggerate about it: way more than half of them are actual violent criminals. It's a violent crime problem this country primarily has, only secondarily a prison-industrial-complex problem.
/jordan