Something has to be done. It's either the case that they are productive members of society, in which case they are rarely imprisoned, or that they are not, in which case they often are.
Children aren't productive members of society, therefore we store them in schools until they've grown up. Same principle - I'm not dehumanizing kids, for the record.
The reason there are more prisoners in America is because there's a larger underclass who are forced out of employment for structural reasons. As long as these issues persist (and indeed, they will get far worse), we have to keep the unemployment rate down somehow. Prisons are just the end effect of this requirement.
The important thing to remember is that the alleged crime doesn't actually matter. This is also why corrupt Wall Street bankers rarely get harsh prison sentences. Why would they, when they're going to be able to find a job without much trouble anyway?
