Why do jails often release criminals from jail?

don't know about were you live but were i live petty crimes are what walk out of jail but to answer your question they are getting out of jail because it is over crowded so they don't have room for people that come in. so first they let out those that's time is almost up within 30 days or so. were i live they try to let those out that are in their for simple drug possession,first time DUI`s and those sitting out fines or those in for misdemeanors. and if you meant prison instead of jail then at least in my state your still wrong most murderers serve at least 10 years of there 20 year terms before their eligible for parole. but most states will put the convict on parole as early as the guidelines of their sentence will allow because just like the jails prisons are to full.