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Footage appeared to show the 32-year-old licking his finger and shining the ball while eating a sweet in the second Test against Australia in Hobart.
He was fined 100% of his match fee by the International Cricket Council (ICC).
"I felt like I've done nothing wrong," he said.
"It's not like I was trying to cheat or anything. I was shining the ball. It's something that all cricketers do.
"Our mouths are always full of sugar. It's such a grey area in the laws of cricket.
"I just ask that everyone gets treated the same way. I think that's fair.
"Obviously the ICC has taken a stance against me, to use me probably as a scapegoat now, but all you can ask for is that everyone gets treated the same."
In 2013, Du Plessis was fined 50% of his match fee for ball tampering in the second Test against Pakistan.
He was caught on camera rubbing the ball against a zip on his pocket in Dubai, with the umpires penalising South Africa five runs and changing the ball. Du Plessis did not contest the charge.
Du Plessis, leading South Africa in the absence of the injured AB de Villiers, has been told to delay any appeal against the latest fine so Cricket South Africa can gather further information from the ICC.
South Africa wrapped up the series 2-0 with an innings-and-80-run win over Australia last week. The final Test in Adelaide starts on Thursday.
"I suppose the thing that's most disappointed us is that we've dominated and played exceptionally well," said Du Plessis.
"That's taken all the shine away from that - excuse the pun."
South Africa captain Faf du Plessis says he has been made a "scapegoat" and denied any wrongdoing after being found guilty of ball-tampering.