A lot of fuss going on about unfair sentences for those involved in the rioting. But consider this: why do people commit a crime? Assuming it's not a crime of passion it's because the following calculation returns true:
chance(caught)*cost(caught) < benefit(crime)
That is, the perceived chance of getting caught combined with the perceived cost of getting caught is outweighed by the benefits of committing the crime. In short, they think this crime would probably pay. It's worth the risk.
During the rioting people perceived that the chance of getting caught was very low and therefore they decided to take the risk. There will come times again when the perceived risk of getting caught is low. In order to stop people committing crimes in those circumstances, the other half of the calculation must be balanced. The cost of getting caught must be higher. There therefore appears to be a perfectly sound reason for giving unusually harsh sentences after the rioting.
Of course, this assumes that the primary purpose of punishment is to deter others. It also assumes that in times of need the requirement that the punishment be proportional to the crime can be relaxed.