There are a number of reasons why Scipio was exonerated while the average infantry soldier was punished (in addition to the banishment to Sicily you referenced they were also refused any back salary for the Cannae campaign). Scipio Africanus was part of the senatorial class and was from the same patrician bloodlines as the men who had been responsible for the strategy and the leadership which had led to the disaster at Cannae. His father had been Consul, and his family was well connected in Rome, and because of this Scipio got a great deal of good press about his military service. Which isn't to suggest his actions were exaggerated, rather that they were presented to the public more openly. He was a young man at the battle of Cannae, only 19 or 20, but he had already famously saved his father's life at The Battle of Ticinus through an act of personal bravery. Further, after the defeat Scipio rallied together a force of 10,000 survivors and apparently forced a group of young nobles at swordpoint to swear an oath to protect Rome at all costs. These stories of personal bravery and leadership meant he was publically perceived as a capable commander who was brave and competent despite the otherwise disastrous battle. There was a sense, on the other hand that the infantry had betrayed Rome by running away. None of the foot soldiers had powerful family in Rome who could advocate on their behalf. The logic in blaming the survivors rather than the leaders seems to have been the same as the reasoning behind decimation, "pour encourager les autres" essentially as a way to set an example for the rank-and-file to discourage desertion.

Goldsworthy, Adrian K. (2007).The Fall of Carthage: The Punic Wars 265-146BC. Cassel.

O'Connell, Robert L. (2010). The Ghosts of Cannae. Random House.