Sonic could have stopped Add if he wanted, but Add's huffiness is amusing enough that he just lets it happen. He doesn't actually need that much cake anyway.

"Stealing is only easier than asking for permission when you're not likely to get caught," he shrugs, stabbing a fork into the cake. He stuffs it into his mouth with a happy hum, his eyes going half-closed. "Not that I'm caught, mind you, but you are sitting right in front of me. It's just good manners."