in the Guardian narrative.
"An initial analysis of the crown court cases suggests the three most severe sentences relating to the riots were handed to individuals who did not directly participate in the disorder, but were convicted of inciting riots via Facebook."
Its because under UK law being an accessory before or after the fact carries the same penalty as the principal offense - that's hundreds of years old so nothing new about that.
You may find that's actually the same in the US too - its associated with conspiracy.
Some details here for the US :
United States
Main article: Aiding and abetting
The U.S. criminal code makes aiding and abetting a federal crime itself a crime:<2>
(a) Whoever aids, abets, counsels, commands, induces or procures the commission of an offense, is punishable as a principal.
(b) Whoever willfully causes an act to be done which if directly performed by him or another would be an offense, is punishable as a principal.
A person may be convicted of aiding and abetting any act made criminal under the code. The elements of aiding and abetting are, generally:
(1) guilty knowledge on the part of the accused ( the mens rea);
(2) the commission of an offense by someone; and
(3) the defendant assisted or participated in the commission of the offense (the actus reus).
http://en.wikipedia.org/wiki/Accessory_%28legal_term%29btw - that's an editorial in the Guardian : not LBN.