Prosecutors are evaluated on the basis of their conviction rate. A plea bargain resulting in probation counts as a conviction, and is easier for the prosecutor to obtain than a jail sentence after a trial conviction.
This needs to change. Somebody who has demonstrated that he is a chronic criminal needs to be excluded from plea bargains. After N arrests (N=3 is a good number, I think), it should be mandatory that the prosecutor go for the max penalty from a trial conviction.
Make that convictions/plea bargains and I agree. An arrest is equivalent to an accusation, and being simply accused shouldn't be the threshold for invoking higher punishment - that is a recipe for a catastrophe, IMO.