>> How could it be worse than doing time?

Ask his lawyer. It is likely not a matter of which is better or worse punishment. It is a matter of risk, risk that one punishment will spin off into others. Someone out on community service, effectively parole, can get in trouble for all sorts of minor things (ie not showing up for work). A kid like this on community service will likely break some rule, resulting in further punishments. Jail time is not only shorter but places less responsibility on the individual. His lawyer might therefor have suggested, given the high likelihood of him breaking rules on community work, that jail time was the safer option.