I see the misunderstanding. You're thinking that the pumps should be designed to always give the receipt afterward. Yes, that would be the ideal way to reduce the possibility of cheating.

However, in the pumps he's talking about, the machine prompts you beforehand: "Click here for a receipt". The receipt decision (yes or no) is always before. There is no way to ask for a receipt afterwards.

He always asks for a receipt (even if he doesn't need the receipt) because he figures that the machine is less likely to cheat him in that case.