From the introductory paragraphs in this story, we can see that the gang's values are based around rather trivial exploits that show their contempt for the normal values of society. They are a gang who stand in opposition to society, but they only express this through meaningless and petty acts of defiance, such as the one mentioned in this quote:
He was late at the rendezvous, and the voting for that day’s exploit took place without him. At Blackie’s suggestion the gang was to disperse in pairs, take buses at random and see how many free rides could be snatched from unwary conductors (the operation was to be carried out in pairs to avoid cheating). They were drawing lots for their companions when T arrived.
If we take this as being symptomatic of the kind of activities that Blackie's gang got up to, we can understand how the action of trying to get free rides without paying for them encapsulates the way in which these boys and the gang itself is built on a foundation of resistance to the normal values of society. They are defined by the position that they take against society and those that form a part of it.