The probation service is not the same as the prison service. People on probation have to check in periodically with their probation officers but they are not under 24 hour observation. Blaming the probation service for the actions of these people is unfair and sidesteps the main problem.
The criminal justice system is under a huge amount of pressure to make space for new prisoners so probation and early release are being used to clear space in prisons. Criminals who pose a threat to society should be in prison, we also need much better rehabilitation processes while they are there, but that is another argument all together.