Avi: Many of the frum Jews in prison don’t belong there. Convictions aren’t necessarily demonstration of actual guilt. (Additionally, actual guilt of a secular crime isn’t always a halachic violation. i.e. if someone is convicted of refusing to testify against someone else.) Innocents are convicted routinely in the non-Jewish system (and guilty parties are routines acquitted.) And of those that do deserve to be there, many are meikel on both bein adam l’makom and bein adam l’chaveiro.
But, as I said, there are exceptions to every rule.