According to the wiki article, the sentences aren't mandatory. They're presented as a choice where one choice is a period of time in jail and the other is an action in the community and a shorter period of time in jail.
IANAL but, I suspect that since the full length jail sentence is considered neither cruel nor unusual and is an option for the convicted person, that should mean that if they choose a different option, it can't be cruel because they found that option more pleasant than one that is already determined to not be cruel. I guess this assumes that people will tend to choose the less cruel punishment given two options, but that seems like a safe assumption.
IANAL but, I suspect that since the full length jail sentence is considered neither cruel nor unusual and is an option for the convicted person, that should mean that if they choose a different option, it can't be cruel because they found that option more pleasant than one that is already determined to not be cruel. I guess this assumes that people will tend to choose the less cruel punishment given two options, but that seems like a safe assumption.