Something odd about this argument is the fact that 30% is pretty much an industry standard. Apple isn't jacking up the price.
I don't see it mentioned (at all), but my hypothesis for 30% is that it is because of money laundering. Otherwise, you wouldn't be able to provide gift cards, would have to limit spending per app, cap all the prices, monitor whale spenders, etc. Easiest just to make it unprofitable.
* iTunes gift cards regularly go on sale for 60-70% of its value on third party markets, and * 30% is a reasonable, to cheap, price to pay for money laundering,
I think your theory is unfounded.