If you hand me two sheets of paper, one of them containing instructions and another containing data, I'll have a pretty easy time keeping them separate, and I think most humans wouldn't struggle with that problem either.
The code sheet says take some bits from the data sheet and interpret it as if it were on the code sheet.
> If you hand me two sheets of paper, one of them containing instructions and another containing data, I'll have a pretty easy time keeping them separate
You think. But there are ways around that. How about a credible extortion message targeting specifically you, that is embedded somewhere on the data sheet? Suddenly, the data has become the instructions...