30% isn't terrible in terms of laundering money, but chances are the App Store payout is how you're citing your income for taxes, so it's more risky than other ventures if Apple were to inform the IRS about something like "90% of this developer's revenue is via gift cards...".
I think also that it's often desirable to run multiple steps. Losing 30% from a whole pipeline is one thing, but losing 30% from one transaction out of a couple seems like a fairly big hit.
