I believe it was a pretty standard error on Tonio's part, but he did cause an avoidable accident and it was too spectacular for the stewards to ignore. They didn't have much option but to issue a 5-place penalty and reprimand: the grid penalty is automatic and the reprimand was probably so that the Hispania people didn't fall off their chairs laughing at getting a non-existent punishment. After all, Tonio was already starting from last in Monza, and it's not like there's much chance of him getting another 2 reprimands by the end of the season (when they all get reset).
This is more a message to stop drivers further up the field from getting ideas than anything else.